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U-pb geochronology and lithogeochemistry of the Hope Bay greenstone belt, Slave structural province,.. Hebel, Manfred U. 1999

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U-PB GEOCHRONOLOGY AND LITHOGEOCHEMISTRY OF THE HOPE BAY GREENSTONE BELT, SLAVE STRUCTURAL PROVINCE, NORTHWEST TERRITORIES, CANADA by MANFRED U. HEBEL B.Sc, Concordia University, 1995 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming -to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1999 © Manfred U. Hebel, 1999 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. 1 further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada Date %\\e | ^ DE-6 (2788) Abstract The Hope Bay greenstone belt (HBGB) is one of several Late Archean greenstone belts recognised within the Slave Structural Province (SSP) in the Northwest Territories, Canada. Unlike most other major greenstone belts in the SSP relatively little is known about the age and evolution of the HBGB. The main goal of this study was to construct a detailed chronostratigraphic and chemostratigraphic framework for the belt that would constrain the temporal and tectonic evolution and thus permit the HBGB to be placed in the regional geological context of the Slave Structural Province. A regional geochronological and lithogeochemical program was carried out in conjunction with geological mapping by BHP Minerals Canada Ltd. personnel. A total of 21 U-Pb age determinations, 174 major and trace element analyses, and 19 rare earth element analyses resulted from the study. U-Pb geochronology was selected as the critical tool for constraining the stratigraphic and temporal evolution of the belt because of its high blocking temperature and the precise ages that can be obtained using this method. Major, trace, and rare earth element data were employed to characterise the geochemistry of various igneous units and investigate the possible paleotectonic settings in which individual units were generated. The HBGB is characterised by a basal series of mafic dominated tholeiitic volcanic flows (Young Group), overlain by a sequence of calc-alkaline volcanic rocks (Westerberg Group), that are in turn overlain by sedimentary rocks of the Tweedy and Farrar group. These sequences were deposited over a period of at least 116 m.y. from ca. 2716 to ca. 2600 Ma. Chemical compositions of volcanic rocks are typified by low abundance of HFSE and depletions in Nb, Ti, Eu, and P relative to REE. The striking similarity between the overall lithologic assemblage and the geochemical signature of volcanic rocks in the HBGB with ii modern arc and back-arc systems (e.g. Mariana and Tonga-Kermadec regions) suggest the HBGB evolved in an arc-backarc geodynamic setting. iii Table of Contents Abstract ii Table of Contents v List of Tables vList of Figures vii Acknowledgements viiChapter 1: Introduction 1 IntroductionMethodsPresentation 3 References 4 Chapter 2: U-Pb Geochronology of the Hope Bay Greenstone Belt, Slave 5 Structural Province, Northwest Territories, Canada Introduction 5 Regional Geology of the Slave Structural Province 5 Geology of the Hope Bay Greenstone Belt 9 Previous Geochronology Studies 13 Analytical Methods 1Geochronology of the Hope Bay Greenstone Belt 18 Stratigraphic Reconstruction of the Hope Bay Greenstone Belt 35 Evolution of the Hope Bay Greenstone Belt 42 Conclusions 44 References 6 Chapter 3: Regional lithogeochemistry of Archean volcanic successions in 50 the Hope Bay Greenstone Belt, Slave Structural Province, N.W.T., Canada iv Introduction 50 Regional GeologyGeology of the Hope Bay Greenstone belt 53 Geochemistry 58 Lithogeochemistry 75 Discussion and Conclusions 8References 88 Chapter 4: Conclusions 92 ConclusionsAppendix I 94 Analytical Precision 9XRFICP-MS 4 v List of Tables Table 2.1 U-Pb analytical data 14 Table 2.2 Regression parameters 7 Table 3.1 Major and trace element data 60 Table 3.2 Rare earth element data 7 Table 3.3 High field strength element ratios 78 Table 5.1 XRF precision 95 Table 5.2 ICP-MS precision 6 vi List of Figures Figure 1.1 Geology of the Slave Structural Province 2 Figure 2.1 Geology of the Slave Structural Province 6 Figure 2.2 Geology of the Hope Bay greenstone belt 10 Figure 2.3 Age determinations for the Hope Bay greenstone belt 19 Figure 2.4 U-Pb Concordia diagrams 21 Figure 2.5 Temporal evolution diagram 36 Figure 2.6 Chronostratigraphy of the Hope Bay greenstone belt 37 Figure 3.1 Geology of the Slave Structural Province 51 Figure 3.2 Geology of the Hope Bay greenstone belt 54 Figure 3.3 Stratigraphy of the Hope Bay greenstone belt 57 Figure 3.4 Binary element ratio diagrams 70 Figure 3.5 Winchester Floyd diagram 3 Figure 3.6 Magmatic affinity plot 74 Figure 3.7 Plot of TiC»2 vs. Zr for the Hayden formation 76 Figure 3.8 REE diagram for the Hayden formation 7 Figure 3.9 REE diagram for the Son Volt formation 81 Figure 3.10 REE diagram for felsic volcanic of the Hope Bay formation 83 vii Acknowledgments This body of work has benefited from the assistance of numerous individuals. First and foremost, I wish to thank my supervisor Jim Mortensen whose guidance in the field and laboratory allowed this effort to come to fruition. A huge debt of gratitude is owed to Richard Friedman whose endless patience and steady hand spared the author much grief in the U-Pb chemistry lab. James Gebert is thanked for sharing his wealth of knowledge on the Hope Bay greenstone belt and who's wry wit provided endless hours of amusement. This work would not be possible without the generous funding provided by BHP Minerals Canada Ltd. Thanks to Greg MacMaster of BHP Minerals Canada Ltd. for his ideas and insights into this project and the mining industry. Thanks are extended to Anna, Darren, Jamie, Tod, Dinger, Scotty, Rob, Terri, Chris, and Steve and all the geologists I've worked and played with. Chris, thanks for allowing me to tap into that huge database of knowledge you possess. Steve thanks for shedding a ray of light into the black box of lithogeochemistry. A special debt of gratitude is extended to Dr. Evil whose perceptions of life and answer to the "meaning of life the universe and everything" proved the Hitchhikers Guide to the Galaxy wrong. Finally, My family: Mom, Dad, Erica, Andrea, Oma, and Opa are thanked for their unwavering support throughout my life. Thanks for the emails Kidz! To my close friends, Dave and George, thanks for everything! Last but not least, thanks A.H! There are no words to describe the amount of support and inspiration you've provided over the years, except, what took you so long? viii Chapter 1 Introduction The Hope Bay greenstone belt (HBGB) lies in the northern portion of the Bathhurst Block in the northeastern part of the Slave Structural Province (Figure 1.1). A regional geochronological and lithogeochemical program was undertaken in conjunction with regional geological mapping of the HBGB in June 1996. The program is entirely funded by BHP Minerals Canada Ltd. and resulted in 21 U-Pb age determinations, 174 major and trace element analyses, and 19 rare earth element analyses. The main thrust of the study was to construct a detailed chronostratigraphic framework for the belt that would constrain its temporal evolution and thus permit the HBGB to be placed in the regional geological context of the SSP. In light of the resulting temporal framework, lithogeochemical results were used to investigate rock compositions and tectonomagmatic affinity of the igneous rock units in the HBGB within this chronostratigraphy. Methods U-Pb geochronology U-Pb geochronology was selected as the main dating tool for this study. High precision age determinations were achieved primarily on zircon (ZrSi02) and to a lesser extent on titanite (CaTiOSiOs) crystals. The robust nature of this method coupled with high blocking temperatures (~ 900°C) of zircon crystals (Lee et al., 1991) results in a powerful method for testing stratigraphic relationships throughout the HBGB. Analyses were carried out by the author at the Geochronology Laboratory of the University of British Columbia. 1 Figure 1.1. Location of the Hope Bay greenstone belt within the Slave Structural Province (modified from Fyson 1997-13). 2 Lithogeochemistry Major and trace element geochemistry were determined for volcanic and plutonic rocks of the HBGB using X-ray fluorescence at Chemex Labs in North Vancouver, British Columbia, using glass beads for majors and pressed pellets for trace elements. A subset of samples were selected for rare earth element analyses using inductive coupled plasma mass spectrometry at Activation Laboratories in Ancaster, Ontario. Resulting data are integrated into the chronostratigraphy, derived from isotopic dating, and used to establish the chemostratigraphy and possible tectonomagmatic affinity of the supracrustal sequences within the HBGB. Presentation This thesis is presented as two research papers (Chapter 2 and 3), to be submitted for publication in refereed journals. These are preceded by introductory comments (Chapter 1) and followed by concluding remarks (Chapter 4). Some care was taken to eliminate redundancies in such topics as regional geology and introductory comments. However, to benefit readers' clarity and continuity a certain amount of repetition is unavoidable. Chapter 2 discusses the geology and U-Pb geochronology of the HBGB. Twenty-one age determinations provide a temporal framework for the evolution of supracrustal secessions. Chapter 3 explores the lithogeochemistry and possible tectonomagmatic setting of igneous units in the HBGB within the chronostratigraphy established in chapter two. Immobile-incompatible trace element ratios are used to characterise individual igneous rock suites and rare earth element abundances are used to suggest potential geodynamic settings. 3 . References Lee, J.W., Williams, I.S., and Ellis, D.J. (1997). Pb, U, and Th diffusion in natural zircon. Nature, v. 390 p 159-162. 4 Chapter 2 U-Pb geochronology of the Hope Bay Greenstone Belt, Slave Structural Province, Northwest Territories, Canada Introduction The Hope Bay greenstone belt (HBGB) lies in the northeastern portion of the Slave Structural Province'(SSP) (Figure 2.1). This mafic dominated greenstone belt is one of several Archean volcanic successions belonging to the Yellowknife Supergroup (YkSG) (Henderson, 1970). The use of high precision U-Pb geochronology in conjunction with geological mapping has provided a temporal framework for the evolution of the YkSG and surrounding plutonic suites (Mortensen et al., 1988; Isachsen et al., 1991. van Breemen et al., 1992). Age determinations indicate the YkSG was deposited between 2715-2655 Ma. However, the preponderance of this data is from the western and central greenstone belts with scant ages from belts in the east. Despite this bias, all dates fall within the temporal framework established from the type section at Yellowknife. The work on the Yellowknife greenstone belt (YkGB) (Mortensen et al, 1988; Isachsen et al, 1991) serves as an excellent example of the level of geochronology needed to unravel the complex history of Archean volcanic successions in the SSP. In an effort to address the dearth of data from eastern belts a detailed geochronolocal study of the HBGB was initiated. The main thrust of this work is to constrain the evolutionary history of supracrustal rocks within the HBGB and in turn place the findings in context of the Yellowknife Supergroup and overall evolution of the SSP. Regional Geology of the Slave Structural Province 5 116 112° 108° 1 ' Figure 2.1. Generalized geology of the Slave Structural Province (modified from Fyson 1997 -13). The following geological overview is drawn largely from prior excellent synopses by: Padgham (1985, 1991), Fyson and Helmstaedt (1988), Hoffman (1989), Padgham and Fyson (1992), Isachsen and Bowering (1994), and King and Helmstaedt (1997). The Slave Structural Province (SSP) located in the northwestern Canadian Shield (Figure 2.1) is a well exposed (= 210 000 km2), dominantly -2715-2655 Ma granite-greenstone-turbidite terrane with subordinate inliers of -4030-2900 Ma gneiss-granite basement rocks, >2800 Ma continental shelf sedimentary rocks, and <2605 Ma polymictic conglomerate-sandstone rocks. The craton is bounded to the west by the -1910-1800 Ma Wopmay orogen and to the east by the -2020-1910 Ma Thelon orogen. To the south, southwest, and northeast the SSP is onlapped by Proterozoic strata. Henderson (1970) initially assigned all supracrustal rocks within the SSP to the Yellowknife Supergroup (YkSG). Workers have since resolved the YkSG into three sequences 1) pre-YkSG: a basal orthoquartzite assemblage, 2) the main greenstone supracrustal sequence (metavolcanic and metasedimentary rocks) and 3) post-YkSG: an upper polymictic conglomerate assemblage. Basement rocks have thus far only been recognised in the western part of the SSP and include a heterogeneous assemblage of orthogneiss, migmatitic gneiss, tonalite, and granodiorite. These rocks are typically strongly metamorphosed, deformed and intruded by amphibolite dykes. The apparent restriction of basement rocks to the western SSP is supported by isotopic boundaries established from sulphide Pb isotopic compositions and whole rock Nd analyses (Thorpe et al.. 1992; Davis and Hegnar 1992; Yamashita et al.. 1995). These data are interpreted to reflect the presence of old sialic basement in the west and an absence of such basement in the east. Pre-YkSG supracrustal rocks are also apparently restricted to the western SSP, where they form a thin discontinuous veneer over crystalline basement rocks. The pre-YKSG units are 7 commonly deformed and comprise mature orthoquartzite with local quartz pebble conglomerate, rhyolitic volcanic rock, chert-magnetite iron formation, siltstone, and calc-silicate rocks locally intruded by ultramafic sills and dykes. Approximately 26 separate granite-greenstone belts are included within the YkSG. These greenstone belts have been further subdivided into mafic-dominated "Yellowknife-type" and a felsic-dominated "Hackett River-type" (Padgham, 1985). Yellowknife-type belts are typically comprised of voluminous massive to pillowed tholeiitic basalt flows (locally variolitic), interleaved with calc-alkaline felsic volcanic and volcaniclastic rocks, turbidites, and local synvolcanic conglomerate and carbonate units. Hackett River-type belts are comprised of calc-alkaline felsic and intermediate rocks intercalated with turbidites in the upper portions of the section. Geochronological age determinations bracket YkSG volcanism between 2715 Ma and 2655 Ma (Mortensen et al., 1988; Isachsen et al., 1991). Volcanic belts are typically isoclinally folded, well-foliated, and cut by belt-parallel shear zones. Metamorphism within the SSP is predominately at greenschist facies although, it locally reaches lower to middle amphibolite facies. A late (<2.6 Ga) sedimentary assemblage consisting of conglomerate and sandstone unconformably overlies the main greenstone sequence. Polymictic conglomerates within this package are typically clast supported, include many lithologies from the volcanic hinterland, and bear a striking similarity with the Timiskaming Group found in the Superior Province (Fyson and Helmstaedt, 1988). Late Archean plutonic rocks in the SSP were intruded between 2.70 and 2.58 Ga (van Breemen et al., 1992; Villeneuve et al., 1997). Villeneuve et al. (1997) further subdivide the intrusive rocks into 2.70-2.64 Ga predeformational tonalite and diorite, 2.62-2.59 Ga K-feldspar megacrystic granite, and postdeformational 2.60-2.58 Ga two-mica granite. Plutonic age determinations thus far indicate a magmatic hiatus from 2.640-2.625 Ga. 8 The SSP shows evidence of three distinct episodes of deformation: 1) >2.8 Ga structures are recorded within the Acasta terrain in the western Slave, but are poorly understood; 2) a pan-SSP deformational event is recorded between 2.7 and 2.6 Ga and characterised by regional compression, local plutonic deformation, and late extension (ca. <2.583 Ga); and 3) 1.84-1.74 Ga brittle to ductile faulting related to the Wopmay and Thelon orogenic belts affect the eastern and western SSP. Geology of the Hope Bay Greenstone Belt The Hope Bay greenstone belt (HBGB) lies in the northern portion of the Bathhurst Block in the northeastern part of the SSP (Figure 2.1). Fraser (1964) first mapped the belt as an Archean volcanic terrane belonging to the YkSG. Gibbons (1986) and Gebert (1990, 1993) classified the supracrustal sequence as a Yellowknife-type belt containing a series of north-south trending linear fractures. The belt is composed primarily of mafic and felsic metavolcanic and subvolcanic rocks, local ultramafic sills and with subordinate metasedimentary rocks. The volcanic belt is surrounded by synvolcanic to postvolcanic granitoid rocks. Metamorphic grade within the HBGB ranges from predominately lower greenschist to amphibolite facies near belt margins. The entire HBGB has been remapped from 1996-1997 by the geological staff of BHP Minerals Canada Ltd. (primarily J. S. Gebert and M.U. Hebel) (Figure 2.2). The revised geological interpretation of supracrustal sequences within the HBGB based on this mapping form the stratigraphic and structural framework for the present study. Informal formation and group names have been assigned by the author to supracrustal rocks within the HBGB to facilitate clarity and continuity for the reader. 9 N W 68"15'H s Legend Lakes Diabase (f~jj) Goulburn Supergroup Hope Ba> formation Granite C^2^ Gneiss C ~} Gabbro Ultramafic Sill Diorite Wilco formation Clover Lake formation Diorite Cf" ~^) Koignuk River formation Windy Lake formation C^3_J) Square Lake formation Wolverine Lake formation 0^^^ Son Volt formation CZZ--"1 Flake Lake formation C^f^) Hayden formation WL- Windy Lake PL- Patch Lake LL- Lynx Lake SL- Square Lake SpL- Spyder Lake Fl- Flake Lake CL- Clover Lake Scale 1:200 000 J L 8km Figure 2.2a. Generalised geology for the northern portion of the HBGB. 10 106°45' Legend Lakes Diabase C^fy Goulburn Supergroup •j/^^i Hope Bay formation Granite ^) Gneiss Gabbro Ultramafic Sill - "Zj§^ Diorite ^) Wilco formation C^^) Clover Lake formation ^j^^1 Diorite CZ^) Koignuk River formation Windy Lake formation J) Square Lake formation Wolverine Lake formation Son Volt formation CZZ-) Flake Lake formation Hayden formation WL- Windy Lake PL- Patch Lake LL- Lynx Lake SL- Square Lake SpL- Spyder Lake Fl- Flake Lake CL- Clover Lake Scale 1: 200 000 106°30' 106"15' Figure 2.2b. Generalised geology for the southern portion of the HBGB. il Mafic rocks are collectively assigned to the Hayden formation (Figure 2.2) and consist of massive and pillowed metabasalt flows and metagabbros. Pillows are commonly variolitic and elongate with length to width ratios ranging from 2:1 up to 10-15:1. Geochemically most mafic rocks are typical Archean tholeiitic basalts although minor magnesium-rich basaltic komatiites have recently been recognised (Lindsey, 1998). Gabbro sills with pod-like geometry are ubiquitous throughout the volcanic sequences, with larger coarse-grained bodies up to 100 metres restricted to the northern portion of the belt. Thick sills commonly grade into more leucocratic differentiates. Ultramafic sills are also found throughout the belt. These sills typically form thin discontinuous intrusive bodies but can reach thicknesses of up to 200 metres in the northern portion of the belt. The HBGB is a mafic dominated volcanic sequence and thus is an exception to the typical felsic dominated greenstone belts found in the eastern SSP. Unlike many Archean greenstone belts, the HBGB also contains a significant component of intermediate rocks. Intermediate units are assigned to the Son Volt formation (Figure 2.2) and were erupted primarily as fragmental rocks although flows occur locally. Felsic volcanic rocks (Flake Lake, Wolverine Lake, Square Lake, Windy Lake, Koignuk River, and Clover Lake formations; Figure 2.2) are dominantly composed of dacitic feldspar-phyric ash and lapilli tuffs with subordinate rhyolitic compositions. Many felsic rocks exhibit volcaniclastic textures suggesting they have been reworked by sedimentary processes. Volcaniclastic rocks are intercalated with flows and felsic epiclastic rocks with rapid facies changes common. Metasedimentary rocks (Wilco and Hope Bay formations; Figure 2.2), including sandstone, siltstone, greywacke, shale, and conglomerate units are interleaved with both mafic and felsic volcanic rocks. 12 Previous Geochronology Studies Bevier and Gebert (1991) reported five U-Pb age determinations from the HBGB prior to this study. These data indicated a minimum age range for felsic volcanism within the belt of 8 (±5) million years (2685 ^ Ma to 2677^ Ma). Synvolcanic and postvolcanic plutons were dated at 2672 Ma and 2608 Ma respectively. The 2608 Ma zircon age was derived from a granite pluton that contained a foliated mafic xenolith; thus providing a minimum age for regional metamorphism. Analytical Methods All U-Pb age determinations reported here were derived from zircons with the exception of a single sample for which an age was obtained from titanite. Most zircon and titanite concentrates were extracted from 20-25 kg samples. Mineral extraction and U-Pb analytical procedures are similar to those described by Mortensen et al. (1995). In order to minimise the effects of surface-correlated Pb loss, all zircon and titanite fractions were strongly abraded prior to analysis (Krogh 1982). Both multigrain and single grain analyses were done. Procedural blanks were 2 to 10 picograms for Pb and 1 to 2 picograms for U. Errors for individual analyses were calculated using the error propagation method of Roddick (1987). The decay constants recommended by Steiger and Jager (1977) were used in age calculations and initial common Pb compositions were estimated using the model of Cumming and Richards (1975). Concordia intercept ages were calculated using either the regression models of Davis (1982) or York (1969) and the algorithm of Ludwig (1980). All age errors are expressed at the 2a level. U-Pb analytical data is given in Table 2.1, and regression parameters are listed in Table 2.2. 13 Table 2.1. U-Pb analytical data for the HBGB. Fraction1 Wt U2 Pb3 206p(j4 Pb5 208pb6 Isotopic ratios (la,%) 7 207Pb/206Pb Ug ppm ppm 204pb Pg % 206pb/238TJ 207pb/235u 207pb/206pb age7 Sample 1 (97PGMC101, Flake Lake rhyolite, 67°46.3'N, 106°16.3'W) A f,Nl,st,3 5 17 10 793 3 13.1 0.5168 (0.17) 13.310(0.23) 0.18679 (0.17) 2714.1 (5.6) B f,Nl,st,3 7 17 10 1861 2 12.8 0.5083 (0.16) 13.078 (0.21) 0.18661 (0.12) 2712.5 (3.8) C f,Nl,st,2 2 11 7 487 2 17.0 0.5217 (0.40) 13.607 (0.45) 0.18916(0.17) 2734.8 (5.6) D f,N 1,81,4 10 56 34 1416 12 13.3 0.5038 (0.08) 12.940 (0.16) 0.18626(0.09) 2709.4 (3.1) E f,Nl,st,4 9 12 7 674 5 13.7 0.5112(0.20) 13.160 (0.28) 0.18672 (0.17) 2713.5(5.6) F f,Nl,st,4 11 41 25 6226 2 13.3 0.5232 (0.12) 13.490 (0.17) 0.18698 (0.08) 2715.8(2.8) Sample 2 (96PTMC119, Wolverine Lake quartz feldspar porphyry, 68°00.7'N, 106°33.7'W) A c,N2,p,i 5 123 70 2148 9 7.2 0.5140 (0.11) 13.075 (0.17) 0.18447(0.09) 2693.5 (3.0) B c,N2,e,p,l 5 61 35 1422 7 8.5 0.5071 (0.21) 13.009 (0.25) 0.18604 (0.10) 2707.5 (3.4) C c,N2,p,l 3 72 41 1642 4 8.3 0.5125 (0.12) 13.096 (0.17) 0.18535 (0.12) 2701.3 (3.8) D m,N2,p,l 18 131 73 7235 10 6.6 0.5124 (0.09) 13.026 (0.15) 0.18435 (0.07) 2692.4 (2.4) E c,N2,p,l 9 83 46 1976 11 6.8 0.5097 (0.12) 12.980 (0.18) 0.18468 (0.09) 2695.3 (2.9) F c,N2,p,t,l 7 93 52 1067 18 7.0 0.5154 (0.09) 13.127 (0.17) 0.18472 (0.10) 2695.7 (3.5) G m,N2,p,t,l 6 123 69 2218 10 8.0 0.5099 (0.11) 12.948 (0.17) 0.18418 (0.08) 2690.9 (2.7) H c,N2,p,l 5 124 70 1117 16 6.9 0.5114(0.10) 13.029 (0.18) 0.18476(0.10) 2696.1 (3.3) I c,N2,p,l 7 129 75 8817 3 10.9 0.5100 (0.09) 12.940 (0.15) 0.18403 (0.07) 2689.6 (2.4) J c,N2,p,l 9 91 51 2621 10 7.0 0.5079 (0.09) 12.913 (0.16) 0.18438 (0.08) 2692.7 (2.7) K c,N2,p,l 6 89 50 3463 5 7.8 0.5047 (0.11) 12.781 (0.17) 0.18366(0.08) 2686.2 (2.6) Sample 3 (96PQMC113, Square Lake tuff, 67°54.1'N, 106°42.1'W) A m,Nl,st,5 23 34 20 2541 10 12.5 0.5037 (0.09) 12.783 (0.16) 0.18405 (0.09) 2689.7 (2.8) B m,Nl,st,5 45 46 28 7379 9 12.7 0.5112(0.10) 12.981 (0.16) 0.18416(0.08) 2690.7 (2.5) C m,Nl,st,3 7 25 15 1842 3 14.9 0.5067 (0.09) 12.860 (0.11) 0.18408 (0.06) 2690.0 (2.0) D f,Nl,e,p,3 5 24 15 1274 3 14.4 0.5026 (0.14) 12.735 (0.22) 0.18378(0.14) 2687.3 (4.6) E f,Nl,e,p,3 7 38 23 4424 2 13.7 0.5148 (0.16) 13.063 (0.19) 0.18403 (0.11) 2689.6 (3.8) F f,Nl,e,p,3 7 37 22 1826 4 13.6 0.5107(0.11) 12.946 (0.18) 0.18385 (0.09) 2687.9 (3.1) Sample 4 (97PQMC109, Square Lake flow, 67 57.2'N, 106°41.6'W) A m,Nl,p,3 8 75 46 2975 6 14.7 0.5089 (0.12) 12.913 (0.18) 0.18403 (0.08) 2689.6 (2.7) B m,Nl,p,4 8 53 32 3633 4 15.1 0.5075 (0.09) 12.869 (0.16) 0.18391 (0.08) 2688.5 (2.6) C m,Nl,p,3 5 51 31 3455 2 14.2 0.5177 (0.14) 13.140 (0.19) 0.18408(0.09) 2690.0 (2.8) D m,Nl,p,2 6 86 53 6375 3 14.7 0.5153 (0.11) 13.075 (0.17) 0.18402 (0.08) 2689.5 (2.6) E m,Nl,p,4 10 44 26 4407 3 14.6 0.4976 (0.11) 12.611 (0.17) 0.18381 (0.09) 2687.6 (2.9) Sample 5 (97PBMC104, Windy Lake tuff, 67°39.7'N, 106°20.5'W) A c,Nl,e,t,5 39 367 195 390 968 10.3 0.4681 (0.15) 11.781 (0.32) 0.18252(0.22) 2675.9 (7.3) B m,Nl,e,t,3 4 227 134 691 37 11.7 0.5098 (0.12) 12.899 (0.22) 0.18353 (0.14) 2685.0 (4.6) C m,Nl,e,t,3 12 352 179 397 267 10.9 0.4465 (0.12) 11.267 (0.29) 0.18300 (0.22) 2680.2 (7.1) D m,Nl,e,t,3 7 232 128 357 123 12.5 0.4742 (0.12) 11.940 (0.32) 0.18261 (0.24) 2676.7 (7.8) E m,Nl,e,t,3 7 190 100 413 8 13.9 0.4436(0.19) 11.175 (0.31) 0.18272 (0.21) 2677.7 (6.9) F m,Nl,e,t,3 6 150 87 919 28 10.8 0.5066 (0.10) 12.818(0.19) 0.18350(0.12) 2684.7 (3.9) Sample 6 (96PTMC123, Windy Lake lapilli, 68°06.3'N, 106°39.3W) A c,N2,e,t,l 23 82 45 8177 7 9.4 0.4956 (0.09) 12.535 (0.15) 0.18345 (0.08) 2684.3 (2.6) B c,N2,e,t,l 8 34 22 2046 4 18.0 0.5143 (0.10) 13.132 (0.17) 0.18520 (0.09) 2700.0 (2.8) C c,N2,e,t,l 4 58 34 1653 4 12.0 0.5124 (0.11) 12.969 (0.17) 0.18357 (0.10) 2685.4 (3.2) D c,N2,e,t,l 7 73 42 3437 4 11.2 0.5090 (0.10) 12.890 (0.16) 0.18366 (0.08) 2686.2 (2.7) E c,N2,e,t,l 11 74 43 1030 23 10.9 0.5076 (0.10) 12.832 (0.18) 0.18334(0.10) 2683.3 (3.4) Sample 7 (97PAMC106, Son Volt lapilli, 67°54.1'N, 106°32.9'W) A m,N2,st,p,l 7 73 42 3939 4 14.9 0.4865 (0.10) 12.301 (0.16) 0.18338 (0.08) 2683.7 (2.6) B m,N2,st,p,l 4 42 25 3112 2 11.7 0.5160(0.12) 13.059 (0.18) 0.18357 (0.08) 2685.4 (2.8) C m,N2,st,p,l 5 139 85 9531 2 14.1 0.5159 (0.09) 13.062 (0.15) 0.18362 (0.07) 2685.8 (2.5) Sample 8 (97PTMC111, Windy Lake quartz-feldspar porphyry, 68°09.5'N, 106°39.4'W) A c,Nl,st,p,3 16 86 49 18512 2 9.9 0.5072 (0.25) 12.812 (0.28) 0.18322 (0.07) 2682.2 (2.4) B c,Nl,st,p,3 27 71 41 15741 4 9.9 0.5126 (0.15) 12.958 (0.19) 0.18334(0.07) 2683.3 (2.4) C c,Nl,st,p,3 20 71 41 19679 2 10.0 0.5161 (0.10) 13.061 (0.15) 0.18355 (0.07) 2685.2 (2.3) Sample 9 (96PCMC108, Koignuk River quartz-feldspar porphyry, 67°34.5'N, 106°35.0'W) A c,N2,e,t,2 18 37 22 2910 7 10.3 0.5089 (0.10) 12.850 (0.16) 0.18314(0.09) 2681.5 (2.9) B c,N2,e,t,l 9 52 30 1374 10 9.4 0.5115(0.11) 12.885 (0.18) 0.18269 (0.10) 2677.5 (3.3) Table2.1 (continued) U-Pb analytical data for the HBGB. Fraction1 Wt U2 Pb3 206p|j4 Pb5 208p[j6 Isotopic ratios (la,%)7 207pb/206pb Mg ppm ppm 204pb Pg % 206Pb/238U 207Pb/235U 207pb/206pb age7 C c,N2,e,t,l 9 50 29 773 17 10.2 0.5111 (0.12) 12.910(0.20) 0.18321 (0.12) 2682.1 (4.0) D c,N2,e,t,2 16 43. 25 2106 10 11.5 0.5105 (0.08) 12.856(0.16) 0.18265 (0.08) 2677.1 (2.7) E c,N2,e,t,l 10 54 32 1466 11 III 0.5138 (0.09) 12.940 (0.17) 0.18266 (0.09) 2677.1 (3.1) F c,N2,e,t,2 12 71 41 5836 5 11.0 0.5093 (0.09) 12.832 (0.15) 0.18275 (0.08) 2678.0 (2.6) G c,N2,e,t,2 11 43 25 2851 5 10.7 0.5088 (0.10) 12.802 (0.16) 0.18250(0.09) 2675.8 (2.8) H c,N2,e,t,4 11 26 15 3357 3 10.9 0.5085 (0.14) 12.799 (0.19) 0.18255 (0.08) 2676.2 (2.7) Sample 10 (96PTMC118, Koignuk River flow, 68°01.3'N, 106°41.3'W) A c,Nl,e,p,3 18 54 32 13147 2 12.6 0.5139 (0.10) 12.949 (0.15) 0.18274 (0.08) 2677.9 (2.7) B c,Nl,e,p,3 • 35 52 31 7525 8 12.7 0.5087 (0.11) 12.804 (0.16) 0.18256(0.08) 2676.3 (2.5) C c,Nl,e,p,3 30 48 28 21174 2 11.7 0.5141 (0.09) 12.946 (0.15) 0.18263 (0.08) 2676.9 (2.5) D m,Nl,e,p,2 6 29 17 1758 3 11.6 0.5041 (0.15) 12.684 (0.20) 0.18248 (0.10) 2675.6 (3.3) E m,Nl,e,p,2 7 16 10 1293 3 11.4 0.5153 (0.12) 12.979(0.19) 0.18266 (0.11) 2677.2 (3.5) Sample 11 (96PCMC107, Koignuk River flow, 67°34.8'N 106°34.7W) A c,N2,e,t,l 13 57 32 1950 12 10.4 0.4980 (0.10) 12.532 (0.17) 0.18252 (0.09) 2675.9 (2.8) B f,N2,e,t,3 50 39 23 4317 14 11.7 0.5025 (0.08) 12.638 (0.15) 0.18241 (0.08) 2674.9 (2.5) C f,N2,e,t,3 50 40 24 4628 13 11.6 0.5078 (0.10) 12.789 (0.16) 0.18266(0.08) 2677.2 (2.5) D c,N2,e,t,l 14 42 24 4422 9 11.1 0.5002 (0.11) 12.602 (0.17) 0.18273 (0.08) 2677.8 (2.6) E f,N2,e,t,4 28 38 22 3300 10 11.0 0.5098 (0.12) 12.839 (0.18) 0.18265 (0.08) 2677.1 (2.7) F f,N2,e,t,l 9 30 18 1232 7 11.9 0.5064 (0.10) 12.815 (0.12) 0.18353 (0.07) 2685.0 (2.4) G f,N2,e,t,l 5 567 328 1405 57 11.0 0.5062 (0.08) 12.742 (0.12) 0.18255 (0.09) 2676.2 (2.9) 1 f,N2,e,t,l 4 54 32 808 8 12.4 0.5039 (0.13) 12.687 (0.16) 0.18262 (0.10) 2676.9 (3.2) J f,N2,e,t,l 3 52 30 1040 5 10.5 0.5028 (0.12) 12.702 (0.14) 0.18323 (0.10) 2682.3 (3.2) H f,N2,e,t,l 4 59 35 1076 7 12.2 0.5145 (0.21) 12.955 (0.22) 0.18264 (0.12) 2677.0 (3.9) Sample 12 (96PBMC104, Koignuk River tuff, 67°32.4'N, 106°15.0'W) A c,N2,e,p,l 5 111 65 2813 6 11.5 0.5089 (0.09) 12.828 (0.15) 0.18284 (0.08) 2678.8 (2.6) B m,N2,e,p,l 3 68 39 538 11 10.9 0.5101 (0.11) 12.833 (0.22) 0.18246 (0.15) 2675.4 (4.9) C m,N2,e,p,l 3 62 36 1143 5 11.1 0.5110(0.12) 12.842 (0.20) 0.18226 (0.12) 2673.5 (3.8) D m,N2,e,p,l 5 63 37 3012 3 11.6 0.5120 (0.17) 12.892 (0.20) 0.18262 (0.13) 2676.9 (4.3) E m,N2,e,p,l 5 33 19 770 7 11.6 0.5016(0.17) 12.664 (0.24) 0.18313 (0.15) 2681.4 (5.0) F c,N2,e,p,l 7 90 52 6103 3 12.5 0.5006 (0.10) 12.616(0.16) 0.18277 (0.08) 2678.2 (2.6) Sample 13 (97PBMC103, Clover Lake rhyolite, 67°40.1'N, 106°27.5'W) A f,N2,e,p,3 20 469 271 625 418 12.0 0.5003 (0.10) 12.473 (0.22) 0.18081 (0.15) 2660.3 (5.1) B f,N2,e,p,l 5 140 66 2054 8 15.9 0.3913 (0.09) 9.640(0.16) 0.17866 (0.09) 2640.5 (3.1) C f,N2,e,s,l 6 307 181 626 81 15.3 0.4908 (0.10) 12.237 (0.22) 0.18082 (0.15) 2660.5 (5.0) D f,N2,e,p,3 12 97 47 1816 14 17.8 0.3943 (0.11) 9.667 (0.18) 0.17784 (0.09) 2632.8 (3.1) E f,N2,e,p,5 22 263 159 1985 82 14.4 0.5093 (0.09) 12.715 (0.17) 0.18106 (0.09) 2662.6 (2.9) Sample 14 (97PTMC108, Sandusky diorite, 67°59.9'N, 106°36.2'W) A f,N2,e,7 32 423 258 14262 26 19.2 0.4839(0.12) 12.033 (0.17) 0.18035 (0.08) 2656.1 (2.6) B f,N2,e,8 40 374 229 11235 36 19.3 0.4862 (0.15) 12.107 (0.19) 0.18060 (0.09) 2658.4 (2.8) C f,N2,e,5 20 425 254 10449 21 18.9 0.4783 (0.10) 11.891 (0.16) 0.18033 (0.07) 2655.9 (2.4) Sample 15 (96PUMC101, Sandusky diorite, 67°54.5'N, 106°31.7'W) A f,N2,e,5 11 350 196 7813 13 14.1 0.4745 (0.09) 11.812 (0.15) 0.18056(0.07) 2658.1 (2.5) B f,N2,e,9 18 246 133 6334 19 11.8 0.4688 (0.08) 11.660 (0.15) 0.18040 (0.07) 2656.6 (2.4) C f,N2,e,14 22 160 85 7416 13 11.1 0.4661 (0.08) 11.586 (0.15) 0.18030 (0.07) 2655.6 (2.4) D f,N2,e,10 25 314 184 2443 84 18.4 0.4715 (0.08) 11.741 (0.16) 0.18062 (0.08) 2658.5 (2.7) E f,N2,e,5 16 430 246 7253 25 17.2 0.4667 (0.11) 11.623 (0.16) 0.18063 (0.07) 2658.7 (2.4) F f,N2,e,6 18 585 343 6038 46 18.7 0.4689 (0.13) 11.681 (0.18) 0.18066 (0.08) 2659.0 (2.5) Sample 16 (T961-43b, foliated g ranite, 67°38.0'N, 106-13.5'W) A cc,N2,e,l 22 50 28 5650 6 7.3 0.5058 (0.10) 12.511 (0.16) 0.17939(0.08) 2647.2 (2.5) B m,N2,e,p,l 5 124 68 3128 6 7.8 0.5020 (0.09) 12.416(0.16) 0.17937 (0.08) 2647.1 (2.6) C m,N2,e,p,l 4 111 66 799 15 15.5 0.4936 (0.28) 12.147 (0.33) 0.17847(0.13) 2638.7 (4.3) D cc,N2,e,p,l 7 500 256 2552 38 3.4 0.4878 (0.14) 12.003 (0.19) 0.17847 (0.10) 2638.7 (3.3) E c,N2,e,p,l 7 328 171 8260 8 6.2 0.4835 (0.08) 1 1.867 (0.15) 0.17799 (0.07) 2634.2 (2.5) F c,N2,e,p,l 7 349 185 11778 6 6.0 0.4904 (0.08) 12.156 (0.15) 0.17979 (0.07) 2651.0(2.4) G ti,m,M20,10 36 124 65 2168 57 5.0 0.4938 (0.11) 11.801 (0.17) 0.17332 (0.09) 2589.9 (2.8) H ti,m,M20,10 81 159 88 1524 231 10.3 0.4940 (0.17) 11.796 (0.22) 0.17317(0.11) 2588.5 (3.6) Sample 17 (96PDMC126, Hope Bay Conglomerate granitoid clast, 68°09.2'N, 106°44.0'W) A m,st,eq,5 13 60 36 8489 3 14.2 0.4980(0.09) 12.656 (0.15) 0.18432 (0.08) 2692.1(2.5) 15 Table2.1 (concluded) U-Pb analytical data for the HBGB. Fraction' Wt U2 Pb3 206p|54 Pb5 208pb6 Isotopic ratios (lrj,%)7 207pb/206pb Mg ppm ppm 204pb Pg % 206pb/238TJ 207pb/235TJ 207pb/206pb age7 B m,st,eq,3 9 64 37 8877 2 13.7 0.4899 (0.14) 12.440(0.18) 0.18416(0.09) 2690.7 (3.0) C m,e,p,4 7 18 11 1489 3 17.3 0.5169 (0.24) 13.201 (0.27) 0.18524 (0.10) 2700.3 (3.5) Sample 18 (96PDMC125, Hope Bay conj domerate granitoid clast, 68°09.2'N, 106°44.0'W) A f,N5,eq,l 9 16 9 1049 4 9.9 0.5140 (0.13) 13.007 (0.20) 0.18352 (0.12) 2684.9 (3.9) B f,N5,eq,l 6 33 19 1404 4 11.6 0.4976 (0.11) 12.656 (0.18) 0.18444 (0.10) 2693.2(3.1) C f,N5,eq,l 4 48 29 889 7 12.6 0.5123 (0.14) 13.001 (0.21) 0.18408 (0.12) 2689.9 (3.9) D f,N5,eq,l 7 87 50 3628 5 11.1 0.5042 (0.09) 12.729 (0.15) 0.18310(0.08) 2681.2 (2.5) E f,N5,eq,l 8 108 57 5293 4 12.1 0.4604 (0.09) 11.515 (0.15) 0.18140 (0.07) 2665.7 (2.5) F f,N5,eq,l 7 65 38 1571 9 14.8 0.4979 (0.09) 12.569 (0.17) 0.18308(0.09) 2680.9 (3.0) Sample 19 (96PDMC128, Hope Bay Conglomerate sandstone, 68°09.2'N, 106°44.0'W) A f,Nl,st,e,l 5 183 103 2777 6 8.5 0.5060 (0.11) 12.514(0.17) 0.17939(0.08) 2647.2 (2.8) B f,Nl,st,e,l 5 112 67 406 9 12.5 0.5152 (0.13) 12.980 (0.28) 0.18273 (0.20) 2677.8 (6.5) C mNl,st,e, 1 26 71 40 5126 11 10.7 0.4912(0.13) 12.289 (0.18) 0.18144 (0.08) 2666.0 (2.5) D c,Nl,st,p,l 20 139 76 6294 13 8.1 0.4978 (0.09) 12.335 (0.15) 0.17972 (0.07) 2650.3 (2.5) E c,Nl,st,e,l 10 187 100 12066 5 6.6 0.4919(0.08) 12.245 (0.15) 0.18055 (0.07) 2657.9 (2.4) F f,Nl,st,e,l 8 199 109 10004 5 7.3 0.4990 (0.08) 12.487 (0.14) 0.18150(0.07) 2666.6 (2.4) G F,Nl,st,e,l 7 96 55 3839 5 11.3 0.4969 (0.10) 12.474 (0.16) 0.18207(0.08) 2671.8 (2.6) Sample 20 (96PK.MC121, Wilco sediments, 67°57.5'N, 106°35.6'W) A c,N2,t,p,l 4 135 88 2072 8 14.1 0.5417(0.10) 14.739 (0.17) 0.19733 (0.08) 2804.3 (2.6) B m,N2,t,l 14 50 30 1265 16 10.7 0.5150 (0.09) 13.328 (0.17) 0.18768(0.09) 2722.0(3.1) C m,N2,e,t,l 8 77 63 2603 9 12.1 0.6553 (0.12) 24.013 (0.17) 0.26577 (0.08) 3281.3 (2.6) D m,N2,e,t,l 8 66 40 1622 10 14.7 0.5114(0.12) 12.864 (0.18) 0.18243 (0.09) 2675.1 (3.0) E m,N2,st,e,l 10 58 34 1939 9 10.8 0.5136(0.09) 13.034 (0.16) 0.18408 (0.09) 2689.9 (2.9) Notes: Analytical techniques are listed in Mortensen et al. (1995). ' Upper case letter = fraction identifier; All fractions air abraded; Grain size, intermediate dimension: cc=>180p.m, c= <180|am to >134p:m, m=<134p.m and >104p.m, f=<104p.m; Magnetic codes:Franz magnetic separator sideslope at which grains are nonmagnetic (N) or Magnetic (M); e.g., Nl=nonmagnetic at 1°; Field strength for all fractions =1.8A; Front slope for all fractions=20°; Grain character codes: ti=titanite, e=elongate, eq=equant, p=prismatic, st=stubby, t=tabular, ;Numeral=number of grains analysed. ^ U blank correction of l-3pg ± 20%; U fractionation corrections were measured for each run with a double 233TJ_ 235,j spike (about 0.005/amu). ^Radiogenic Pb ^Measured ratio corrected for spike and Pb fractionation of 0.0043/amu ± 20% (Daly collector) and 0.0012/amu ± 7% and laboratory blank Pb of lOpg ± 20%. Laboratory blank Pb concentrations and isotopic compositions based on total procedural blanks analysed throughout the duration of this study. ^Total common Pb in analysis based on blank isotopic composition ^Radiogenic Pb ^Corrected for blank Pb, U, and common Pb. Common Pb corrections based on Cumming and Richards (1975) at the age of the rock or the 207Pb/206Pb age of the fraction (errors are 2a in Ma) 16 Table 2.2. Regression parameters. Sample No. Fractions Upper Intercept (Ma) Lower Intercept (Ma) Probability of fit (% or MSWD) Regression method 1 (97PGMC101) A,B,D,E,F 2716.3 !2g 38l!f27 91 Davis 2(96PTMC119) A,D,F,G,I,K 2698.7 ^'7 909^4 96 Davis 3 (96PQMC113) A,B,C,D,E,F,G 2690.2 70 Davis 4 (97PQMC109) A,B,C,D,E 2690.0 ^'5 166 !2^2 96 Davis 5 (97PBMC104) A,B,C,D,E,F 2685.2 133^ 56 Davis 6-(96PTMC123) A,C,D,E 2685.8!, .5 101+243 -258 44 Davis 7-(97PAMC106) A,B,C 2685.6 !j'68 „,+122 85-126 82 Davis 8 (97PTMC111) A,B,C 2685.1 ^ 397+soo JY ' -426 65 Davis 9(96PCMC108) B,D,E,F,G,H 2677.9 ^;48 9,„+796 260 _920 83 Davis 10(96PTMC118) A,B,C,D,E 2677.4 +_[95 -,,,+388 226-373 97 Davis 11 (96PCMC107) A,B,C,D,E,G,H,I 2677.2!" 01 +253 81 -269 81 Davis 12(97PBMC103) A,C,E 2662.7 !32 g 1 {,1 +332 167-359 67 Davis 14(97PTMC108) A,B,C 2663.2 !}Q'J 269+M1 ZOY-136 0.96 Modified York II 15 (96PUMC101) A,B,C,D,E,F 2662.4 +_™ 135+413 -419 1.39 Modified York II 16(T961-43B) A,B,C,D,E 2649.5 !22 5 651-132 49 Davis 17(96PDMC126) A,B,C 2701.1 ^ 438+179 41 Davis 18-(96PDMC125) A,D,E,F 2685.9 !2 3 3„7 +65 JYL -65 68 Davis 17 Geochronology of the Hope Bay Greenstone Belt U-Pb analytical data are presented for 20 samples from the HBGB. Sample locations with age determinations are shown in Figure 2.3. Zircon morphologies are generally similar in all dated samples and range from equant grains to slender acicular grains with length to width (l:w) ratio of 5:1. Short stubby prisms were the dominant morphology observed. Low U (<100 ppm) content zircons were the norm with few elevated U levels encountered (100-567 ppm). Volcanic and hvpabyssal rocks Sample 1 (97PGMC101, Flake Lake fm) - quartz-eye rhyolite A massive quartz-eye rhyolite flow cross cut by gabbroic dykes, bounded by gabbro on the west, basalt on the south, and granite on the north and eastern flank was sampled from a glacially striated outcrop on the eastern shore of Flake Lake in the southern portion of the HBGB (Figure 2.3b). Zircons separated from this sample are relatively non-magnetic and form a homogeneous population consisting of colourless, pale pink, subhedral, stubby grains, 80-120 microns in length with a length to width ratio of -2:1. Most zircon grains were broken, probably during the mineral separation process. There were no visible cores or igneous zoning and rare bubble-shaped inclusions were present. Measured U concentrations were low (<57 ppm, Table 2.1). Five strongly abraded multigrain fractions range from 0.1 to 3.6 % discordant and define a chord with upper and lower concordia intercepts of 2716.3*26 Ma and 381 Ma (Figure 2.4a) respectively. The upper intercept is interpreted as the crystallisation age of the rhyolite with the relatively young lower intercept indicating a recent Pb loss event. Fraction C lies to the right of the discordia curve with a 207Pb/206Pb age of 2734.8 ±5.6 Ma and is 18 N 106°45' I 106"30' s Legend C_ ^) Lakes ^Hfc Diabase Goulburn Supergroup Hope Bay formation Granite (^^) Gneiss CZff) Gabbro ^^fe Ultramafic Sill Diorite Wilco formation C J) Clover Lake formation Diorite Koignuk River formation ~y Windy Lake formation Cffff) Square Lake formation Wolverine Lake formation Son Volt formation Flake Lake formation C_ ^) Hayden formation WL- Windy Lake PL- Patch Lake LL- Lynx Lake SL- Square Lake SpL- Spyder Lake FI- Flake Lake CL- Clover Lake Scale 1:200 000 2 4 6 8km 106°30' I Figure 2.3a. Age determinations for the Northern portion of the HBGB. All ages derived from zircon crystals. 19 Scale 1: 200 000 Legend (^fff) Lakes Diabase Goulburn Supergroup Hope Bay formation C^~\) Granite C^ff) Gneiss Gabbro Ultramafic Sill ) Diorite C~^) Wilco formation C^ff) Clover Lake formation Diorite (ff^) Koignuk River formation Windy Lake formation Square Lake formation Wolverine Lake formation Son Volt formation ^ Flake Lake formation CZZ^1 Hayden formation WL- Windy Lake PL- Patch Lake LL- Lynx Lake SL- Square Lake SpL- Spyder Lake FI- Flake Lake CL- Clover Lake 8km J 6/30' 106"30' I 10615' Figure 2.3b. Age determinations for the Southern portion of the HBGB. All ages derived from zircon crystals with the exception of one age derived from titanite (t). 20 0.52 (a) Sample 1 (Flake Lake fm. Rhyolite) 2716.3+3.1/-2.6 Ma 275.0/ X PL, 207Pb/235U 0.531 15.0 0.50 0.46 (c) Sample 3 (Square Lake fin. Tuff) 2690.2+3.6A2.8 Ma ,7Pb/235U 12.0 13.0 14.0 0.50 0.42 (e) Sample 5 (Windy Lake fin. Ttuff) 2685.2 +2.97-2.5 Ma 2700 P /F . % 2600/-PL, >^ § 2550 >^ / 207Pb/235U 10.5 12.5 0.51 0.491 (b) Sample 2 (Wolverine Lake fm. Dome) / 2698.7 +6.1/-3.7 Ma S • £ 00 2700/ ^ A 2650/p1^ /A / V $7 B 207Pb/235U 12.0 13.0 0.54 0.52 0.50 0.48 (d) Sample 4 (Square Lake fin. Flow) 2690.0+2.0/-1.5 Ma 12.0 13.0 0.52 0.50 0.48 14.0 14.0 (f) Sample 6 (Windy Lake fm. Lapilli) 2720/^ 2685.8 +2.97-2.5 Ma / P 2680X^ X PL, s 2640/ X / A /• !/k 207Pb/235U 12.0 12.8 13.6 Figure 2.4. U-Pb concordia plots for rocks of the HBGB (samples 1-6). 21 Figure 2.4. (continued) U-Pb concordia plots for rock of the HBGB (samples 7-12). 0.54 0.46 0.38 (m) Sample 13 (Clover Lake fm. Rhyolite) 2662.7 +3.47-2.8 Ma • £ CO g 2500/ o 2600 ^/E • 2400/^ /B £- 1 1 207Pb/235U 9.0 11.0 13.0 (o) Sample 15 (Diorite) 268( 2662.4 +18.7/-13.1 Ma 2640 0.48 0.50 0.46 (q) Sample 17 (Hope Bay fm. Clast) / 2701.1 H6/-3.8 Ma S P 2700/ OH / O \/ /ft / 2600/ / / /A f / / V/ <?/ 207Pb/235U 1.0 13.0 15.0 0.52 0.48 0.44 (n) Sample 14 (Diorite) 2663.2 +15.1/-10.1 Ma OH Pb/2"U 14.0 0.51 0.49 0.47 0.53 OH (p) Sample 16 (foliated granite) Crystallization age 2649.5 +2.9A2.5 Ma Metamorphic age 2589.4 +2.2A2.2 Ma 2640. fA SB 2600/ / rf 207Pb/235U 1.0 12.0 13.0 0.49 0.45 (r) Sample 18 (Hope Bay fm. Clast) . 2685.9+2.47-2.3 Ma / co 2700/^ . HO ° 2600^ 2650^4 •2550/^ 47 207Pb/235U 11.0 13.0 Figure 2.4. (continued) U-Pb concordia plots for rocks of the HBGB (samples 13-18). 23 0.52 (s) Sample 19 (Hope Bay fin. Sandstone) < 2647.2+2.8A2.8 Ma 27utv 0.50 2600/ 2650/ 0.481 'B "W'Pb ages of A) 2647.2 +2.8/-2.8 Ma B) 2677.8+6.5/-6.5 Ma C) 2666.0+2.5A2.5 Ma D) 2650.3 +2.5A2.5 Ma E) 2657.9 +2.4A2.4 Ma F) 2666.6 +2.4A2.4 Ma G) 2671.8+2.6A2.6 Ma 7Pb/235U 11.4 12.6 13.8 0.67 0.57 0.47 (t) Sample 20 (Wilco fin. Turbidite) y*c p 3100/ S 3000 2900/ 2800/ /A 2700/ ft mPb/™Pb ages of A) 2804.3+2.6/-2.6Ma B) 2722.0+3.1/-3.1 Ma C) 3281.3 +2.6A2.6 Ma D) 2675.1 +3.0/-3.0 Ma E) 2689.9+2.6A2.6 Ma 207Pb/235U 11.0 21.0 Figure 2.4. (concluded) U-Pb concordia plots for rocks of the HBGB (samples 19-20). 24 interpreted to have included an older inherited component. This interpretation is supported by the anomalously high proportion of 208Pb in the analysis (17 % 208Pb vs. 12.8-13.7 % 208Pb in the other four fractions; Table 2.1). Sample 2 (96PTMC119, Wolverine Lake fm) - quartz-feldspar porphyry This unit crops out in the northern portion of the HBGB south of Patch Lake (Figure 2.3a) where it intrudes a volumous sequence of basaltic flows. Zircons recovered from this sample show a uniform morphology, comprising subhedral to euhedral, clear to somewhat turbid, moderately fractured, pale coppery pink prismatic crystals with ubiquitous broken terminations. Minor bubble and rod-like inclusions were present and no visible cores or zoning are apparent. Eleven single grains of similar morphology and size were analysed (Table 2.1) with degrees of discordance ranging from 0.7-2.8 %. The data show considerable scatter (Figure 2.4b) that is thought to reflect the effects of an older inherited zircon component in at least some of the grains. Six analyses define a linear array that bound the left side of the data array, resulting in upper and lower intercepts of 2698.7 ^ Ma and 908 Ma respectively. The upper intercept age gives a maximum crystallisation age for the quartz-feldspar porphyry. Sample 3 (96PQMC113, Square Lake fm) - feldspar phyric felsic tuff The feldspar phyric tuff sample was collected from a small glacially polished outcrop north of Square Lake and 1000 meters east of the western bordering granites (Figure 2.3a). Extensive lichen cover precludes conclusive interpretation of lithological relationships between adjacent units. Tuffaceous fragments exclusively comprise quartz phyric porphyry. Heavy mineral separates contained abundant gem quality equant, multi-faceted zircons with few rod 25 and bubble shaped fluid inclusions. Zircons formed a bimodal size distribution predominately consisting of 100-134 micron and subordinate 134-160 micron grains, with no visible zoning or inherited cores. Three fractions from each population were analysed (Table 2.1) and range from slightly to moderately discordant (0.6-2.8%). A linear regression through all fractions yields an upper intercept, interpreted as the crystallisation age for the tuff, of 2690.2 Ma and a relatively young lower intercept age of 89 Ma suggesting a recent Pb-loss event (Figure 2.4c). Sample 4 (97PQMC109, Square Lake fm) - quartz-feldspar phyric dacite flow The sample was collected near the western margin of the belt on the northeastern shore of Lynx Lake (Figure 2.3a) from a dacitic flow interbedded with volcaniclastic rocks and intruded by a gabbro pod. Zircon separates formed a homogenous population of gem quality, 100-134 micron, pale pink, euhedral, and sharply faceted translucent prisms with a length to width aspect of 2:1. No visible cores or igneous zoning were noted. Five low U fractions were analysed (Table 2.1) and results included one concordant and four variably discordant (0.5-3.8%) analyses. The concordant zircon yielded a 207Pb/206Pb age of 2690.0!22 88 Ma and a linear regression through all grains gives upper and lower intercept ages of 2690.0 Ma and 166 Ma respectively (Figure 2.4d). The upper intercept age is interpreted as the best estimate for the crystallisation of the flow. Sample 5 (97PBMC104, Windy Lakefm) - dacitic tuff? This sample was collected along the eastern shore of Spyder Lake (Figure 2.3b) from an area of rubbley frost-heaved rock. This unit has been strongly carbonate altered, and the intensity of the metasomatism precludes assigning a definite rock type, although the abundant feldspar laths in a fine-grained matrix are similar to a dacitic tuff unit found further north along 26 strike. The sample yielded abundant -180 micron, euhedral, moderately fractured cloudy pale brown zircons. The grains form a single zircon population comprised of multifaceted grains with no visible apparent cores or igneous zoning. Measured U contents of six analysed grains (Table 2.1) are relatively high (-150-367 ppm), and the analyses form a discordant linear array (1.3-13.9 %) (Figure 2.4e). Calculated upper and lower concordant intercepts are 2685.2 ^ Ma and 133 Ma respectively. The upper intercept age is interpreted as the crystallisation age of the feldspar phyric flow. Sample 6 (96PTMC123, Windy Lake fm) - dacitic volcaniclastic lapilli This sample located north of Windy Lake from the northern portion of the HBGB (Figure 2.3a) consists of a volcaniclastic lapilli unit intercalated with suspected turbidite sediments. The sample matrix is weakly carbonate altered and strongly feldspar phyric, and consists largely of angular monolithic lapilli sized quartz phyric felsic fragments. Recovered zircon prisms form a homogenous population of pale pink, translucent, stubby, sharply faceted prisms. No apparent igneous zoning or inherited cores were detected. Five single zircon analyses (Table 2.1) range from 0.9-4.9 % discordant. If fraction B is omitted from the data array the remaining fractions yield a regressed interpreted crystallisation age determination of 2685.812,5 Ma (Figure 2.4f). Fraction B is interpreted as an inherited xenocrystic zircon with a 207pb/206pb age of 2700.0!2288 Ma. Sample 7 (97PAMC106, Windy Lake fm) - dacitic lapilli This sample was collected from a volcaniclastic unit 6 km southeast of Lynx Lake in the central portion of the HBGB (Figure 2.3a). Fragments are monolithic and comprise lapilli sized angular quartz-feldspar porphyry clasts hosted in a felsic fine-grained crystalline matrix. Prior 27 to sample processing, the matrix and contained fragments were separated in an attempt to determine ages of fragments and matrix separately; however only a small number of poor quality, highly fractured zircons unsuitable for analysis were recovered from the matrix. A small quantity of gem quality zircons was recovered from the fragments. The translucent pale pink grains are characterised by slightly elongate (l:w of 2.5:1) stubby prisms with abundant fractures and minute bubble-shaped fluid inclusions. Three single grains were selected for analyses (Table 2.1). One analysis is 5.6% discordant whereas two fractions (B and C) are nearly concordant (0.2 %) with 207Pb/206Pb ages of 2685.4+2288 Ma and 2685.8 +2255 Ma (Figure 2.4g) respectively. A linear regression through all analyses results in an upper concordia intercept of 2685.6 lj | Ma and a lower concordia intercept of and 85 Ma. The upper intercept age is interpreted as the crystalline age for the quartz-feldspar porphyry component of this unit and provides a minimum crystallisation age for the volcaniclastic unit. Sample 8 (97PTMC111, Windy Lake fm) - quartz-feldspar porphyry Sample 8 located east of Hope Bay (Figure 2.3a) is from a massive quartz-feldspar phyric rhyodacite porphyry body that is overlain and flanked by abundant felsic volcaniclastic rocks. Zircons recovered comprise a single population of -150 micron, multifaceted stubby prisms with few bubble-shaped inclusions. The gem quality grains showed no evidence of igneous zoning or inherited cores. One concordant and two slightly discordant (0.7-1.7%) fractions were analysed (Table 2.1) and define a linear array with calculated upper and lower concordia intercepts of 2685.1 *\20 Ma and 397 Ma respectively (Figure 2.4h). Fraction C is nearly concordant (0.1 %) with a 207Pb/206Pb age of 2685.2 % Ma. The former upper intercept age is interpreted as the age of crystallisation. 28 Sample 9 (96PCMC108, Koignuk River fm) - quartz feldspar dome A subvolcanic feeder to a rhyodacite flow (sample 11) is located along the western margin of the southern portion of the greenstone belt (Figure 2.3b). Heavy mineral separates contained abundant clear, gem quality zircons forming two size populations. The dominant group is comprised of 74-105 micron (l:w of 2:1), slender, pale pink, euhdral, sharply faceted, tabular prisms with minute rod and bubble-shaped fluid inclusions. Grains from the subordinate population were 134-160 microns in diameter, translucent, stubby (l:w of 2:1), pale pink, and generally free of inclusions. No visibly apparent igneous zoning or inherited cores were recognised in either group. Eight fractions, including three single grain fractions, were analysed (Table 2.1). The analyses Pb/ Pb ages ranged from 0.2-1.2 % discordant. Fractions A and C yield slightly older 207Pb/205Pb ages (Table 2.1) then the remaining six analyses and are thought to have incorporated a minor inherited component. A regression through the remaining six analyses yields with and lower concordia intercepts of 2677.91^ 4 Ma and 260 Ma respectively (Figure 2.4i). The upper concordia intercept is considered the crystallisation age of the porphyry stock. Sample 10 (96PTMC118, Koignuk River fm) - dacitic flow This sample was collected west of Patch Lake (Figure 2.3a) where it forms a spatially restricted lava flow that is intercalated with abundant volcaniclastics and intruded by a quartz feldspar porphyry stock. The sample contains parallel oriented feldspar laths hosted in a massive fine-grained matrix. Heavy mineral concentrates yielded abundant gem quality zircons. Two main size populations are present; the main population consists of -250 micron (l:w of 3:1), multifaceted, euhedral translucent pale pink grains, whereas the subordinate group is characterised by -100-134 micron (l:w of 2:1), multifaceted, translucent pale pink prisms. Both 29 groups include a small number of bubble-shaped inclusions with no visible cores or igneous zoning. Five fractions were selected representing both populations (Table 2.1). Three analyses are concordant with the remaining two 1.2 % and 2.0 % discordant. The concordant fractions 207pb/206pb ages are 2677.9 !22, Ma (fraction A), 2676.9 +255 Ma (fraction C), and 2677.2+33;55 Ma (fraction E) yielding an average 207Pb/206Pb age of 2677.3 +2, Ma. A linear regression through all data points results in upper and lower corcordia intercepts of 2677.4 !| 5 Ma and 226 Ma respectively (Figure 2.4j). The upper intercept is interpreted as the best estimate of the crystallisation age for the dacitic flow. Sample 11 (96PCMC107, Koignuk River fm) - rhyodacidic flow This sample was collected from a quartz-feldspar phyric flow located along the southwestern margin of the HBGB (Figure 2.3b). The unit flanks a larger intrusive felsic porphyry dome (sample 9) and is intercalated with flow breccia and fine-grained sediments. Zircons similar in appearance to those recovered in sample 9 were recovered from the felsic flow. Ten fractions representing both grain size populations, including seven single grain fractions, were analysed (Table 2.1). One single grain (fraction H) yielded a concordant analysis with a 207Pb/206Pb age of 2677.0 +_] \ Ma. The remaining grains were variably discordant (1.0-3.2 %). Fractions F and J yield marginally older 207Pb/206Pb ages (Table 2.1) then the remaining eight analyses and are thought to have incorporated a minor inherited component. A linear regression omitting fractions F and J constrain an age of 2677.2 +fx \ Ma (Figure 2.4k). Sample 12 (96PBMC104, Koignuk River fm) -felsic tuff? 30 A fine to medium-grained unit tentatively interpreted to be a felsic tuff occurs in the southern portion of the belt (Figure 2.3b) where it is intercalated with sediments. A moderate amount of zircon was extracted from this sample. Two distinct size populations were recovered. Both size populations were multifaceted, tabular, pale pinkish brown crystals with no visible zoning or cores. Minute bubble and rod-shaped inclusions were noted. Populations were divided based on their grain sizes; 100-134 microns (l:w Of 2:1) and 180-200 microns (l:w of 3:1) respectively. Six single grain fractions were analysed (Table 2.1). Individual analyses were 0.5-2.8 % discordant with 207Pb/206Pb ages ranging from 2673.5-2681.4 Ma (Figure 2.41). Despite the apparent variation in ages all fractions are within analytical error and thus a 907 90(i weighted average of the Pb/ Pb ages of 2677.6 + 1.9 Ma is interpreted as the age of deposition of this unit. Sample 13 (97P BMC 103, Clover Lake fm) - massive rhyolite This sample, located on the northern shore of Clover Lake (Figure 2.3b), is from a small isolated outcrop bordered by turbidites along the eastern and western flanks. The rhyolite consists of rare quartz eyes hosted within a black massive matrix. A single zircon population of ~100 micron tabular pale pink prisms with commonly broken terminations was recovered from the sample. Minute rare elliptical clear inclusions are present and grains are of moderate clarity. Five analyses (Table 2.1) range from 0.4-22.7% discordant. A regression (excluding the two highly discordant fractions) gives upper and lower intercepts ages of 2662.7 +_2t Ma and 167 Ma respectively (Figure 2.4m). The upper intercept age is interpreted as the crystallisation age for the rhyolite. 31 Plutonic rocks Sample 14 (97PTMC108, Diorite) - coarse grained diorite A small outcrop of megacrystic diorite occurs in the northern portion of the HBGB east of Lynx Lake (Figure 2.3a). Copious quantities of high U (Table 2.1) acicular prismatic monocrystalline zircons were recovered from this sample. Reddish pink tabular zircons are elongate (l:w of 5:1) and of moderate quality with highly fractured terminations. No apparent igneous zoning or inherited cores were noted. Three fractions (Table 2.1) representing the highest quality material available were moderately discordant (4.7-6.2 %). The data define a discordia array with imprecise upper and lower concordia.intercept ages of 2663.2 *\50 \ Ma and 135 Ma respectively (Figure 2.4n). The Pb/ Pb ages are within analytical error of each other, hence the weighted average Pb/ Pb age of 2656.7 ± 1.5 Ma for the three fractions should represent the minimum crystallisation age of this rock. The upper intercept age is considered the best estimate for the age this unit. Sample 15 (96PUMC101, Diorite) - coarse grained diorite A diorite intrusion similar to sample 14 crops out in the centre of the supracrustal rock package and is crosscut by granite dykes (Figure 2.3b). Sample 15 is a medium-grained sheared diorite from this locality. . Zircons in this sample are identical in appearance to those from sample 14. Six fractions representing the best material were selected for analyses (Table 2.1). The data are strongly discordant (7.0-8.6 %) resulting in a discordia array (Figure 2.4o) with upper and lower intercept ages of 2662.4 ^J®7 Ma and 269 Ma respectively. The individual 907 *)C\(~\ Pb/ Pb ages for the six fractions (2655.6-2659.0 Ma) are within analytical error of each 007 *)0/X other, thus the weighted average for all Pb/ Pb ages (2657.7 ± 1.9 Ma) should represent the 32 minimum crystalline age for this unit. The upper intercept age is imprecise but is considered to be the best estimate for the crystallisation age of the rock. Sample 16 (T961-43b) - foliated migmatitic granite Sample 16, collected by P.H. Thompsen, is from a foliated migmatitized granite extracted from the large area of plutonic rocks that bounds the HBGB in the southeast (Figure 2.3b). Heavy mineral separates yielded abundant zircon and titanite. Zircons comprise dominantly -150 micron pale pink translucent prisms and subordinate -300 micron elongate (l:w of 3:1) tabular grains. The grains were of excellent quality, and no apparent zoning or cores were detected. Pale brown wedge-shaped titanites of moderately good quality formed a homogenous population. Six single zircon grains were analysed (Table 2.1) and are variably 707 70£ discordant (0.4-4.2 %) (Figure 2.4p). Fraction F yields anomalously old Pb/ Pb age and is thought to contain a minor inherited component; omitting this fraction from a linear regression gives upper and lower concordia intercepts of 2649.5 Ma and 651 Ma respectively. The upper intercept age is considered the crystallisation age for the granite. Multigrain titanite fractions are concordant with overlapping 207Pb/206Pb ages of 2589.9 !2 8 Ma and 2588.5 Ma. The weighted average of the 207Pb/206Pb age of 2589.4 !2 2 Ma for the two titanite analyses gives the best estimate for the timing of peak metamorphism that affected this section of the HBGB. Sedimentary rocks Sample 17 (96PDMC126, Hope Bay fm) - granitoid clast This rounded cobble was collected from a clast supported polymictic conglomerate with coarse-grained sandstone interbeds found on an island within Hope Bay located in the northern 33 tip of the HBGB (Figure 2.3a). Heavy mineral separates yielded a minute amount of homogenous, pristine, euhedral, multifaceted, stubby zircon with few rod-like inclusions. Three low U (<65 ppm) fractions with no apparent cores or zoning were analysed (Table 2.1) and define a discordant (0.8-9.1 %) linear array with upper and lower intercept ages of 2701.1 +f3\ Ma and 438 Ma (Figure 2.4q) respectively. The upper intercept is considered the crystallisation age of the granitoid clast. Sample 18 (96PDMC125, Hope Bay fm) - granitoid clast Sample 18 is from the same polymictic conglomerate unit as sample 17 (Figure 2.3a). The rounded granitoid cobble yielded a small quantity of stubby, monocrystalline, equant, and multifaceted zircons. Facets are slightly rounded, no zoning or inherited cores were detected, and small rod-shaped fluid inclusions were present in a few grains. Six single grains were analysed (Table 2.1) and are variably discordant (0.5-10.1%). A regression through four of six analyses gives upper and lower intercepts of 2685.9*f2\ Ma and 397 Ma (Figure 2.4r). The upper intercept is interpreted to be the crystallisation age of the rock. Two other analyses, fractions B and C give anomalously old 207Pb/206Pb ages of 2691.4Ma and 2689.9 +33;99 Ma, respectively and are considered to represent an older inherited zircon component. Sample 19 (96PDMC128, Hope Bay fm) - coarse-grained sandstone This sample is from a coarse-grained sandstone interbedded within a polymictic conglomerate (hosts sample 17 and 18) sequence on a small island located in Hope Bay (Figure 2.3a). A heterogeneous collection of stubby to prismatic grains of moderate to excellent clarity was recovered from heavy mineral concentrates. Seven single, high quality grains were selected to represent the various grain morphologies present. Analyses (Table 2.1) ranged from 34 concordant to 4.1 % discordant with 207Pb/206Pb ages ranging from 2677.8!" Ma to 2647.2 ^I Ma (Figure 2.4s). The youngest age is considered to represent the maximum depositional age for this sequence. Sample 20 (96PKMC121, Wilco fm) - quartz-feldspar rich greywacke The sample is from a small isolated outcrop of greywacke found in the central portion of the volcanic sequence east of Lynx Lake (Figure 2.3a). Grain morphologies recovered from heavy mineral concentrates are diverse ranging from slender acicular prisms to short, stubby, and rounded grains. Zircons selected for single grain analyses (Table 2.1) were of excellent quality with rare rod and/or bubble-shaped inclusions present and no visible cores or igneous 907 90ft zoning was noted. Slightly discordant analyses (0.6-2.0 %) with Pb/ Pb ages (Figure 2.4t) of 2675.1 ± 3.0 Ma, 2689.9 ± 2.9 Ma 2722.0 ± 3.1 Ma, 2804.3 ± 2.6 Ma, and 3281.3 ± 2.6 Ma indicate a highly diverse sediment source and a maximum depositional age for the unit of 2675.1 ±3.0 Ma. Stratigraphic Reconstruction of the HBGB Twenty-one U-Pb ages reported here, together with results of U-Pb dating studies by other workers, provide a relatively complete temporal and stratigraphic framework for the HBGB (Figure 2.5 and 2.6). Data indicate felsic volcanism spanned a period of at least 53 millions years (2716-2663 Ma) suggesting that the bulk of supracrustal rocks in the HBGB was deposited over that period. This is strikingly similar to the depositional time frame for rocks of the YkGB (2715-2655 Ma). The apparent clustering of ages (Figure 2.5) may indicate that the magmatism was episodic; however, the age differences between the magmatic events are too 35 2730 4 I • Granite 2650 - • Hope Bay fm ® Wilco fm 2640 A Diorite O Clover Lake fin 2630 - • Koignuk River fm 0 Windy Lake fm 2620 -• Square Lake fm • Wolverine Lake fm o Flake Lake fm i Figure 2.5. Plot of age determinations with associated errors for volcanic (O), plutonic (A), and sedimentary rocks (O) within the HBGB. 36 Koignuk River fin (2677 Ma) Flake Lake fin (2716 Ma) Hayden fin (>2716 Ma) Figure 2.6. Generalized stratigraphic column for the HBGB. 37 small to preclude essentially continuous magmatism over the 53 million years in the HBGB. If the ages of the sedimentary units are considered, the temporal framework of the HBGB supracrustal sequences span a period of at least 69 m.y. (>2716 Ma to < 2647 Ma). Although the exact number of mafic cycles in the HBGB remains uncertain (Chapter 3), mafic volcanics within the HBGB have been collectively assigned to the Hayden formation. Directly determining the age of mafic volcanics is difficult. Lack of suitable material to date within sequences requires the use of indirect age information to constrain the depositional age of these units. However, two age determinations provide some preliminary temporal constraints for this formation. To the southwest of Patch Lake (Figure 2.3a), the 2699 Ma Wolverine Lake formation clearly crosscuts mafic volcanic rocks and west of Flake Lake (Figure 2.3b) pillow tops face toward the 2716 Ma Flake Lake formation suggesting the bulk of the Hayden formation is pre-2716 Ma. The oldest felsic unit discovered thus far, the 2716 Ma Flake Lake formation is a poorly exposed rock suite spatially restricted to an area around Flake Lake (Figure 2.3b). The stratigraphic position of the formation within the upper portion of the Hayden formation suggests it was deposited during the waning stages of mafic volcanism, and possibly represents the differentiated product of a high level Hayden formation mafic magma or the partial melting of mafic or felsic rocks. This age is similar to other units across the SSP that define the onset of volcanism in the YkSG. The chemical signature (Chapter 3) of the suite may be of economic significance, as it is known to host VMS occurrences in the Superior Structural Province (Barrie et al. 1993;Thurston 1981; Lesher et al. 1986). The Hayden and Flake formations together comprise the Young Group, which is analogous to the Kam Group in the Yellowknife greenstone belt. 38 The Son Volt formation, a thick unit of intermediate volcanic rocks outcrops southwest of Spyder Lake (Figure 2.3b) with thin discontinuous lenses extending north up to the central portion of the belt. The precise age of this formation is uncertain as geochronological sampling of intermediate rocks failed to yield zircons. However, the Son Volt formation appears to lie stratigraphically above the mafic volcanic rocks and locally contains rare flows of variolitic pillow basalt. This relationship suggests that the Son Volt formation was erupted at the end of the main period of mafic volcanism ca. 2.7 Ga. Deposition of the Son Volt formation was followed by an apparent period of magmatic quiescence which lasted ~ 17 m.y. and culminated in the emplacement of the 2699 Ma Wolverine Lake formation, a series of quartz- and plagioclase-phyric high level intrusions and domes. This formation appears to be spatially restricted to an area along the south western flank of Patch Lake (Figure 2.3a). However, zircon xenocrysts found in the younger Windy Lake formation north of Windy Lake (Table 2.1, Sample 6,) and on the eastern shore of Spyder Lake (Bevier and Gebert, 1991) yield 207Pb/206Pb ages of 2700 and 2701 Ma, respectively, suggesting that rocks comparable in age to the Wolverine Lake formation may be present along the length of the HBGB. Another apparent period of magmatic quiescence which lasted ~9 m.y followed emplacement of the Wolverine formation. This was in turn followed deposition of the aerially restricted Square Lake formation at 2690 Ma in west-central HBGB (Figure 2.3a). The Square Lake formation is intruded to the west by granites, to the northeast by gabbro, and fault bounded by mafics along its northeastern flank. A further apparent volcanic hiatus (~5 m.y.) was followed by deposition of the 2685 Ma Windy Lake formation, one of two dominant felsic suites found throughout the eastern HBGB. The formation is bordered by turbidites on the west, by west facing mafic volcanic rocks along the northeastern flank, and east facing mafics along the southeastern flank. The 2685 Ma volcanic event was followed by another short apparent hiatus (~7 m.y.) in magmatism, which culminated in deposition of the other regionally extensive 39 felsic suite, the 2677 Ma Koignik River Formation found throughout the western HBGB. The unit locally (at sample site 10) crosscuts west facing mafic pillow volcanic flows and is intruded by local gabbro bodies. A final apparent hiatus (14 m.y.) between felsic volcanism events culminated in the extrusion of the Clover Lake formation at 2663 Ma, bordered on either side by turbidites. The Son Volt, Wolverine Lake, Square Lake, Windy Lake, and Koignuk River comprise the Westerberg group, which is correlative to the Banting formation of the YkGB. Typical SSP turbidites (greywacke-mudstone units) are found throughout the HBGB and commonly underlie lakes and valleys due to the recessive weathering of these units. Sedimentary rocks interpreted to be turbidites are collectively assigned to the Wilco formation. Detrital U-Pb work on a sample of turbidite from the central HBGB resulted in a span of 207pb/206pb ageg of. 3281 Ma^ 28Q4 Ma^ 2?22 Ma^ 269Q Ma^ and 26?5 Ma Thege dateg provide a maximum depositional age of 2675 Ma for this unit. More interesting are two pre-YkSG ages (2804 and 3281 Ma) derived from single zircon grains that showed no morphological evidence of prolonged transport (i.e. rounding or frosting). These ages are, however, common in other parts of the SSP. Although current models of the SSP suggest rocks of this age are restricted to the western SSP, old zircons within the Hope Bay turbidites could have been derived from a source that lay further east or these zircons may also suggest a more proximal source concealed somewhere within the Bathurst Block. Further support for the presence of basement within the Bathurst block comes from the geochemical signatures of selected Hayden formation mafics (chapter 3). Rare earth element depletions (Nb, P, and Ti) on a primitive mantle normalised diagram indicate these melts may have assimilated sialic crust (Chapter 3) (Kerrich and Wyman, 1997; Barley, 1986). A more refined age constraint for the Wilco formation may be provided by the 2663 Ma Clover Lake formation. The position of this suite, which is bounded by turbidites on either side, suggests it was erupted shortly before or possibly during the deposition of the sedimentary 40 rocks. The age of this unit indicates the depositional age of the Wilco formation may be ca. 2660 Ma, which is very similar to the 2661 Ma for the Burwash Formation (Bleeker and Villeneuve, 1995) of the YkSG. The Wilco and Clover Lake formations together comprise the Tweedy group. The upper sequence in the HBGB also contains a variety of coarse-grained clastic rocks collectively assigned to the Hope Bay formation. A polymictic conglomeratic unit located in the northern portion of the HBGB (Figure 2.3a) unconformably overlies the greenstone belt and is considered to be the youngest member of the supracrustal package. Prior workers (Padgham, 1996) dated a single clast from this unit and reported an age of 2715 Ma for the cobble. Two granitic cobbles extracted from this unit and dated in this study yield ages of 2701 Ma and 2686 Ma, respectively. The 2686 Ma age suggests plutonism was occurring at the same time as volcanism during the deposition of the Windy Lake Formation. Detrital U-Pb work from a sandstone interbed found within this unit suggest a maximum age of deposition for this unit was 2647 Ma. However, this conglomerate texturally resembles the <2.6 Ga "Jackson Lake" type conglomerate, and therefore may be of similar age. Despite the homogenous age for this formation, units comprising this formation demonstrate dramatic internal facies changes along strike and it is uncertain whether this represents facies variations of a single unit or if they represent separate units of different ages. Relatively few U-Pb age determinations for plutonic rocks are available from the Bathurst block of the SSP. U-Pb dates for three plutonic rocks in the HBGB reported in this study, together with two prior age determinations (Bevier and Gebert, 1991) indicate that most and perhaps all of the plutonic rocks within the Bathurst Block are either coeval with or younger than the supracrustal rocks of the HBGB. Age data for plutonic rocks within the Bathurst Block is still too limited to preclude the possibility of older crystalline basement in this region. The few dates have shown the existence of synvolcanic (2672 Ma, ca 2663 Ma, and 2650 Ma) and 41 sydeformational (2608 Ma) plutonism. In addition, if granitic cobbles hosted within the Hope Bay formation are locally derived then synvolcanic plutonism extends from 2701-2650 Ma. A foliated orthogneiss unit borders the southeastern contact of the belt. Titanite interpreted to be metamorphic in origin was extracted from this unit, and dated at 2589 Ma. This is believed to represent the time of peak of regional metamorphism in this area. However, a granite intrusion dated on the northwest side of the HBGB which contains foliated mafic xenoliths suggests the lower age limit for the peak of regional metamorphism and deformation of the HBGB is 2608 (Bevier and Gebert, 1993). A similar paradox is observed by Villeneuve et al. (1997) with the relatively undeformed 2602 Ma Chin Lake stock of the Anialik River greenstone belt and the nearby <2600 Ma String Lake conglomerate which displays a strong penetrative foliation. Villeneuve et al. (1997) suggest this apparent contradiction may result from differing local strain rates within the regional deformation events or suggests the terminal deformation event was episodic. Evolution of the HBGB Archean greenstone belts have commonly been considered to contain numerous volcanic cycles, exhibiting a regular progression from mafic to felsic compositions. Cyclical volcanism is repeated to produce stacked sequences commonly in excess of ten kilometres in thickness. This process was believed to occur over a relatively short timeframe and in close proximity to the present distribution of the volcanic sequences. The volcanic successions are thought to result from three main processes: 1) tapping and differentiating from subvolcanic magma chambers, 2) interleaving of distal deposits of one volcano with the proximal deposits of another volcano, and 3) tectonic thrusting and interleaving of volcanic strata (Sylvester et al., 1997). 42 The HBGB appears to be typical of such sequences; geochronological, geochemical (Chapter 3), and structural data suggest mafic-felsic sequences likely resulted from all three mechanisms. The HBGB appears to contain a simple stratigraphy (Figure 2.2 and 2.6) with an older unit of dominantly mafic volcanic rocks (Young group), a middle unit of dominantly felsic rocks (Westerberg group) of various ages, and a younger unit of sedimentary rocks (Tweedy and Farrar group) derived from the erosion of mafic, felsic and plutonic rocks. However, age determinations in concert with local younging directions suggest structural complexity. Within the HBGB there are several areas where older units overly younger lithologies: 1) In the northern HBGB, west younging mafics are bordered to the east by younger turbidites underlying Windy Lake (Figure 2.2a) and in the southern portion of the belt turbidites underlying and along strike of Spyder Lake may define a major thrust fault spanning the entire length of the belt. 2) On the northeast side of Spyder Lake a thrust fault appears to have placed older mafic volcanic rocks upon 2685 Ma felsic volcanic rocks (Figure 2.2b), 3) Mafic volcanics are also suspected to be thrusted upon turbidites to the west of Clover Lake (Figure 2.2b). Hence, the occurrence of turbidites within the HBGB may be used to infer the top of the volcanic pile. Volcanism within the HBGB spanned a period in excess of 53 m.y. and if the Hope Bay formation is synchronous to the Jackson Lake Formation of the YkSG then the age framework for supracrustal rocks of the HBGB exceeds 100 m.y. In light of the complex structural relationships present and protracted evolutionary history for the belt, assigning a definitive depositional history for the belt is fraught with numerous pitfalls. Despite the complexities, the prolonged temporal framework, evidence of thrust faults, and contrasting geochemical sugnatures of the mafic (MORB and ocean island signatures) and felsic volcanic rocks (arc signature) can be reconciled in an island arc/back-arc geodynamic setting. With the tholeiitic Hayden and Flake Lake formations deposited in a back-arc basin, respectively preceding the deposition of the overlying Son Volt and calc-alkaline Wolverine Lake, Square Lake, Windy 43 Lake, Koignuk River, and Clover Lake formations were deposited in the fore-arc (chapter 3). The overlying sedimentary successions were then deposited after the mafic and felsic rocks were brought together during formation of the SSP. The back-arc/island arc system proposed for the HBGB is consistent with the one proposed for the YkGB by Helmstaedt and Padgham (1986) and given our present level of knowledge of these granite-volcanic-turbidite terranes appears to best explain the formation of the main phase of the YkSG in the SSP. Placing the findings of this study into the context of the overall evolution of the SSP is a precarious task. At our present level of understanding; with key elements necessary to formulate a tectonic model likely removed during the Thelon and Wopmay orogens and dubious lithological relationship, models explaining the evolution of the SSP fall tentatively under two main doctrines: collisional and extension tectonics. However, proponents of any one model are hard pressed to reconcile the protracted history of the SSP (4.0-2.6 Ga) coupled with the inherent diversity of rocks that comprise the SSP. Hence, workers are increasingly gravitating to a multi-stage tectonic model invoking a hybrid of the two end member beliefs (Isachsen and Bowering, 1994; Bleeker et al., 1997; Bleeker and Ketchum; 1998). Conclusions Twenty-one U-Pb age determinations for rocks of the HBGB help constrain volcanism, plutonism, sedimentation, and deformation within the belt. Volcanism occurred over an interval of at least 53 million years, beginning with the deposition of mafic rocks, the bulk of which are thought to have been deposited in a back-arc basin prior to circa 2716 Ma. Calc-alkaline felsic volcanism deposited in an island arc setting occurred from 2716 Ma to 2663 Ma and is separated by suspected volcanic hiatuses. Sedimentation of turbidites ensued volcanic activity at ca. 2663 44 Ma and was in turn followed by a protracted depositional hiatus of ca 60 m.y. culminating with the deposition of the Hope Bay formation at ca. 2600 Ma thus marking the top of the supracrustal sequence. Plutonic activity is marked by synvolcanic (2699 Ma, 2672 Ma, 2663 Ma, and 2650 Ma) and syndeformation (2608 Ma) intrusive rocks. Titanite cooling ages from a gneiss bordering the southeastern margin of the HBGB suggest the peak of regional metamorphism occurred at ca. 2589 Ma. The temporal framework for the HBGB is strikingly similar to the YkGB, as both supracrustal sequences appear to have been deposited from >2716 Ma to <2600 Ma. This serves to reinforce the dominance of the YkSG throughout the entire SSP as originally proposed by Henderson (1970). Geochronological constraints indicate supracrustal successions in the HBGB are complicated by thrust faults and considering their protracted evolutionary history (»100 m.y) support workers claim that greenstone belts are not the result of a single magmatic or tectonic event but rather represent episodic volcanism and sedimentation (Isachsen and Bowering, 1994; Sylvester et al, 1997; Bleeker et al., 1997; Bleeker and Ketchum; 1998). 45 References Barley, M.E. (1986). Incompatible-element enrichment in Archean basalts: A consequence of contamination by older sialic crust rather than mantle heterogeneity. Geology v. 14, p 947-950. Barrie, C.T., Ludden, J.N., and Green, T.H. (1993). Geochemistry of volcanic rocks associated with Cu-Zn and Ni-Cu deposits in the Abitibi subprovince. Economic Geology v.88, p 1341-1358. Bevier, M.L., and Gebert, J.S. (1991) U-Pb geochronology of the Hope Bay-Elu Inlet area, Bathurst Block, Northeastern Slave Structural Province, N.W.T. 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Gebert, J.S. 1990. Geology of the Hope Bay and Elu Inlet metavolcanic belts, Bathurst Block, northwestern Slave Province, N.W.T. Abstracts in Exploration Overview 1990, N.W.T., 26 p. Gebert, J.S., 1993. Geology and mineral potential of the Archean Hope Bay and Elu Inlet volcanic belts, Northeastern Slave Structural Province, District of Mackenzie, NWT. Northern affairs program Northwest Territories Geology Division EGS paper 1993-1, 103 p Gibbons, W.A., (1987). Preliminary geology of the central Hope Bay volcanic Belt, northern portions of NTS 77 A/3,6 (map with marginal notes, 1:50 000 scale) DIAND, EGS 1987-12. Helmstaedt, H. and Padgham, W.A. (1986). A new look at the stratigraphy of the Yellowknife Supergroup at Yellowknife, NWT- Implications for the age of gold-bearing shear zones and Archean basin evolution. Canadian Journal of Earth Sciences v. 25, p 454-475. Henderson. J.B. (1970). Stratigraphy of the Yellowknife Supergroup, Yellowknife Bay-Prosperous Lake area, District of Mackenzie, Geological Survey of Canada, Paper 70-26. Hoffman, P.F. (1989). Precambrian geology and tectonic history of North America. In A.W. Bally and Palmer, A.R. (eds), The Geology of North America-An Overview. Geological Society of America, Boulder, Colorado, p 447-512. Isachsen, C.E., Bowring, S.A., and Padgham, W.A. (1991). U-Pb zircon geochronology of the Yellowknife Volcanic Belt, NWT, Canada: new constraints on the timing and duration of greenstone belts magmatism. Journal of Geology v. 99, p 55-67. Isachsen, C.E., and Bowring, S.A. 1994. Evolution of the Slave: Geology, v.22, p 917-920. King, J.E., and Helmstaedt, H. (1997). The Slave Province, North-West Territories, Canada. In M.J. De Wit, and L.D. Ashwal. (eds), Greenstone Belts. Oxford Monographs on Geology and Geophysics; no. 35. p 459-479. Kerrich, R. and Wyman, D.A. (1997). Review of developments in trace-element fingerprinting of geodynamic settings and their implications for mineral exploration. Australian Journal of Earth Sciences, v. 44, p 465-487. Krogh, T.E. (1982). Improved accuracy of U-Pb zircon ages by creation of more concordant systems using an air abrasion technique. Geochimica et Cosmochimica Acta, v. 46, p 637-649. Lesher, CM., Goodwin, A.M., Campbell, I.H., and Gorton M.P. (1986). Trace element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada. Can. J. Earth Sci. v. 23, p 222-237. 47 Ludwig, K.R. (1980). Calculations of uncertainties of U-Pb isotope data. Earth and Planetary Science Letters, v. 46, p 212-220. Lindsey, D. (1998) Geochemical and Petrographic Study of the Mafic-Ultramafic Suite in the Hope Bay Volcanic Belt, Slave Structural Province, NWT. B.Sc. thesis, University of British Columbia 55 p. Mortensen, J.K., Thorpe, R.I., Padgham, W.A., King, J.E., and Davis, W.J. (1988). U-Pb zircon ages for felsic volcanism in the Slave, N.W.T. Radiogenic Ages and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 88-2 p 85-95. Mortensen, J.K., Ghosh, D.K., and Ferri, F. (1995). U-Pb geochronology of intrusive rocks associated with copper-gold porphyry deposits in the Canadian cordillera. In Porphyry deposits in the Northwestern Cordillera of North America. Edited by T.G. Schroter, G-2, p 491-531. Padgham, W.A. (1985). Observations and speculations on supracrustal successions in the Slave Structural Province. In L.D. Ayres, P.C Thurston, K.D. Card, and W. Weber (eds), Evolution of Archaean Sequences. Geological Association of Canada. Special Paper 28, p 133-151. Padgham, W.A. (1991). The Slave Province, an overview; Geological Survey of Canada, Open File 2168, p 1-40. Padgham, W.A., and Fyson, W.K. (1992). The Slave Province: a distinct Archean craton. Canadian Journal of Earth Sciences, v.29, p 2072-2086. Padgham, W.A. (1996). Slave conglomerate dating. Northern affairs program Northwest Territories Geology Division EGS paper 1996-12, 85 p. Roddick, J.C. (1987). Generalised numerical error analysis with applications to geochronology and thermodynamics. Geochemica et Cosmochimica Acta, v.51, p 2129-2135. Steiger, R.H., and Jager, E. (1977). Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science letters, v.36,p 359-362. Sylvester, P.J., Harper, G.D., Byverly, G.R., and Thurston, P.C. (1997). Volcanic aspects. In M.J. De Wit, and L.D. Ashwal. (eds), Greenstone Belts. Oxford Monographs on Geology and Geophysics; no. 35. p 55-90. Thorpe, R.I., Cumming, G.I., and Mortensen, J.K. (1992). A significant Pb isotope boundary in the Slave Province and its probable relation to ancient basement in the western Slave Province. In Project summaries: Canada-Northwest Territories Mineral Development Subsidiary Agreement Geological Survey of Canada, Open File 2484, p 179-164. 48 Thurston, P.C. (1981). Economic evaluation of Archean felsic volcanic rocks using REE geochemistry. In Archean geology (ed J.E Glover and D.I. Groves), Special publication No.7, Geological Society of Australia, p 439^450. van Breemen, O., Davis, W.J., and King, J.E. (1992). Temporal distribution of granitoid plutonic rocks in the Slave Province, Northwest Canadian Shield, Canadian Journal of Earth Sciences v.29, p 2186-2199. Villeneuve, M.E., Henderson, J.R., Hrabi, R.B., Jackson, V.A., and Relf, C, (1997). 2.70-2.58 Ga plutonism and volcanism in the Slave Province, District of Mackenzie, Northwest Territories: in Radiogenic Age and Isotopic Studies: Report 10; Geological Survey of Canada, Current Research 1997-F, p 37-60. Yamashita, K., Jensen, J.E., Creaser, R.A., and Gebert, J.S. (1995). Geology, geochemistry, and Nd isotopic study of the Hanikahimajuk Lake area (NTS 86 1/2, 86 H/14,15), northern Point lake belt, Slave Stuctural Province, NWT; Geological Association of Canada, Program with Abstracts, v.20, p A-l 12. York, D. (1969). Least squares fitting of a straight line with correlated errors; Earth and Planetary Science Letters, v. 5, p 320-324. 49 Chapter 3 Regional lithogeochemistry of Archean volcanic successions in the Hope Bay Greenstone Belt, Slave Structural Province, N.W.T., Canada Introduction The Hope Bay greenstone belt (HBGB) is one of several Late Archean greenstone belts recognised within the Slave Structural Province (SSP) (Figure 3.1). Unlike most other major greenstone belts in the SSP relatively little is known about the age and evolution of the HBGB. The preponderance of data from these granite-turbidite-volcanic sequences come from studies focussing on belts from the western and central portion of the SSP. However, in light of recent research, the nature and character of the HBGB is rapidly developing. Geological mapping (Gebert, 1993) and high precision U-Pb geochronology (Chapter 2) provided key constraints and allowed supracrustal sequences to be placed into a preliminary stratigraphic sequence. This paper examines the lithogeochemical evolution of the HBGB in the context of the chronostratigraphy established in Chapter 2. Major, trace, and rare earth element data are employed to characterise the geochemistry of various igneous units and explore the possible paleotectonic settings in which individual units were generated. Regional Geology of the SSP The Slave Structural Province (SSP) in the northwestern Canadian Shield (Figure 3.1) has a long and complex history, and contains rocks that record approximately half the history of 50 Figure 3.1. Generalized geology of the Slave Structural Province (modified from Fyson 1997-13). 51 the Earth (King and Helmstaedt, 1997). Henderson (1970) originally assigned all supracrustal rocks of the SSP to the Yellowknife Supergroup (YkSG). Recent geochronology studies have shown the YkSG can be further sub-divided (i.e., pre and post-YkSG), although the stratigraphy has not yet been formally revised (King and Helmstaedt, 1997). The oldest rocks in the SSP (4.0-2.8 Ga) are gneisses, granitoids and minor supracrustal rocks (pre-YkSG) that are recognised thus far only in the western half of the province (Padgham, 1985, 1991). These rocks are thought to represent an older cratonic basement which is referred to as the Acasta terrane after the 4.02 Ga Acasta gneiss (Bowering et al., 1990; Stern and Bleeker, 1997). Younger granitoid intrusions in the western SSP have Sm-Nd-Pb isotopic signatures that indicate they were contaminated by this older terrane (Thorpe et al., 1992; Davis and Hegner 1992; Yamashita et al., 1995). In contrast, younger granitoids of the eastern SSP have isotopic signatures that indicate they have not interacted with older basement rocks. This observation is generally interpreted to indicate that older basement rocks are absent from the eastern SSP. With minor exceptions, the bulk of the volcanic rocks of the SSP were deposited in a 60 m.y. period between 2715 and 2655 Ma (YkSG) (Isachsen and Bowering, 1994; Mortensen et al., 1992). Greenstone belts have been subdivided into mafic-dominated Yellowknife-type belts, and intermediate to felsic dominated Hackett River-type belts (Padgham, 1985, 1991). Some belts also contain minor synvolcanic conglomerate and carbonate units (e.g., Lambert et al., 1990). Voluminous turbidites were deposited throughout the SSP at the end of volcanism. In many greenstone belts turbidite deposition began during the waning stages of volcanism, and felsic volcanic rocks interfinger with the turbidites. Geochemical studies of the turbidites suggest they were derived from both felsic volcanic rocks and continental crust of granodioritic composition (McLennan and Taylor, 1.984). This indicates that early synvolcanic intrusions and pre-existing basement rocks were uplifted and eroded to provide detritus. 52 After cessation of volcanism and deposition of the turbidites, the craton experienced a period of apparent magmatic quiescence (2640 to 2625 Ma) (van Breemen et al., 1992). Magmatic activity resumed at approximately 2625 Ma and continued until the peak of regional metamorphism and deformation at ca. 2590 Ma. A series of young conglomerates ("Jackson Lake-type") containing abundant granitoid clasts was deposited after ca. 2600 Ma. These young conglomerates record late uplift along reactivated faults and are similar to the Timiskaming Group of the Superior Province. Geology of the Hope Bay Greenstone belt The Late Archean HBGB (Figure 3.1) lies entirely within the fault-bounded Bathurst Block that forms the northeast portion of the SSP. The HBGB comprises a north-trending supracrustal package approximately 90 kilometres long and 15 to 20 kilometres wide. Fraser (1964) first mapped the belt in a 1:506,880 project known as Operation Bathurst. Gibbins (1987) mapped the northern portion of the HBGB (NTS 77 A/3, 6) and a team led by Gebert (1993) mapped the remainder of the belt (NTS 76 0/8, 10, 15, 16 and 77 A/2, 3, 6, 7, 10). Reconnaissance U-Pb geochronology studies by Bevier and Gebert (1991) constrained the timing of felsic volcanism between 2685-2677 Ma and plutonism from 2672-2608 Ma. The entire HBGB has been remapped from 1996-1997 by the geological staff of BHP Minerals Canada Ltd. (primarily J. S. Gebert and M.U. Hebel) (Figure 3.2). The revised geological interpretation of supracrustal sequences within the HBGB based on this mapping form the stratigraphic and structural framework for the present study. Informal formation and group names, assigned by the author, have been assigned to supracrustal rock in the HBGB (Chapter 2) to facilitate clarity and continuity for the reader. 53 N CD CD s Legend C_ ~y Lakes ^^^^ Diabase Goulburn Supergroup Hope Bay formation Granite Gneiss Gabbro Ultramafic Sill Diorite Wilco formation Clover Lake formation Diorite CD5 Koignuk River formation CD) Windy Lake formation Square Lake formation ^^^^ Wolverine Lake formation Son Volt formation CD Flake Lake formation Hayden formation WL- Windy Lake PL- Patch Lake LL- Lynx Lake SL- Square Lake SpL- Spyder Lake FI- Flake Lake CL- Clover Lake Scale 1:200 000 2 4 6 8km 106 45' I Figure 3.2a. Generalised geology for the northern portion of the HBGB. 54 106°30' Legend CZZ^ Lakes >^Hfe Diabase Scale 1:200 000 Goulburn Supergroup Hope Bay formation Granite ^) Gneiss (ZZ) Gabbro Ultramafic Sill ' j Diorite CZZ^1 Wilco formation C^Z^ Clover Lake formation l^^fc Diorite C_Z) Koignuk River formation -^y1 Qoy Windy Lake formation Square Lake formation ^^J> Wolverine Lake formation Son Volt formation C_Z) F'ake Lake formation CZZ) Hayden formation WL- Windy Lake PL- Patch Lake LL- Lynx Lake SL- Square Lake SpL- Spyder Lake Fl- Flake Lake CL- Clover Lake 8km J Figure 3.2b. Generalised geology for the southern portion of the HBGB. 55 The stratigraphy of the HBGB (Figure 3.3) is composed of the basal mafic dominated Young group, comprised of the Hayden and Flake Lake formations, respectively. Felsic volcanic and hypabyssal rocks of the Westerberg group overlie the Young group, which is in turn overlain by sedimentary rocks of the Wilco and Tweedy groups, respectively. Deposition of supracrustal sequences (Figure 3.2 and 3.3) within the HBGB appears to have occurred over a period in excess of 100 m.y. (>2716 - <2600 Ma) (Chapter 2). Volcanism spanned an interval of at least 53 million years, beginning with the deposition of the mafic Hayden formation, the bulk of which was likely deposited prior to ca. 2716 Ma. Mafic volcanic rocks are commonly pillowed and are interlayered with massive flows and associated gabbroic sills. Variolitic textures are common in pillowed flows of the northern portion of the belt. Most mafic rocks are typical Archean tholeiitic basalts although magnesium-rich basaltic komatiites have recently been recognised (Lindsey, 1998). The Hayden formation is cut by ultramafic intrusions that form sills up to 200 metres in thickness. The 2716 Ma Flake Lake formation, dominantly consisting of rhyolitic volcanic flows occurs near the top of the Hayden mafic pile. Unlike many Archean greenstone belts, the HBGB also contains a significant component of intermediate rocks. Intermediate units, collectively assigned to the Son Volt formation, were erupted primarily as fragmental rocks (tuff and lapilli tuff) although rare flows occur locally. Felsic volcanism (Flake Lake, Wolverine Lake, Square Lake, Windy Lake, Koignuk River, and Clover Lake formations; Figure 3.2 and 3.3), separated by several volcanic hiatuses, occurred from 2716 Ma to 2663 Ma. Ash and lapilli tuffs of dacitic and rhyolitic composition make up the majority of the felsic volcanic rocks. Many felsic rocks exhibit volcaniclastic textures suggesting they have been reworked by sedimentary processes. Sedimentation of turbidites (Wilco formation) followed volcanic activity at ca. 2663 Ma and was followed by a protracted depositional hiatus of ca. 60 m.y., culminating with the deposition of the fluvial sediments (Hope Bay formation) at ca. 2600 Ma. Sedimentary rocks 56 o H u <u p is a, 3 O IH a c 3 „ri - I Clover Lake Wilcofm | (2663 Mai Koignuk River fin (2677 Ma) Windy Lake fin (2685 Ma) Square Lake fin (2690 Ma) Flake Lake fin (2716 Ma) Haydenfin(>2716Ma) Figure 3.3. Generalized stratigraphic column for the HBGB. 57 are poorly exposed and underlie many of the valleys within the belt. The upper stratigraphy of the HBGB includes a series of conglomeratic units, some of which are suspected to record late uplift and erosion of the belt along fault scarps. Geochronological constraints locally demonstrate older over younger relationships and indicate supracrustal successions in the HBGB are complicated by thrust faults (Chapter 2). The HBGB is bordered on the east by plutonic rocks, which include granodiorite, tonalite and gabbro. A granodiorite to the northeast of the HBGB has given a U-Pb zircon age of 2672 +4/-1 Ma (Bevier and Gebert, 1991), indicating these rocks are part of an early, synvolcanic intrusive suite (van Breemen et al., 1992), possibly analogous to tonalite-trondjhemite-granodiorite (TTG) suite in the Superior Province. The southeastern contact of the belt is bordered by a heterogeneous gneiss terrane that includes both migmatised turbidites and numerous granitoid intrusions. Zircons from a granitic orthogneiss within this unit yield a U-Pb age of 2649.5 +2.5A2.5 Ma and a U-Pb titanite age (thought to record the age of metamorphism and deformation of this unit) of 2589.4 +2.2/-2.2 Ma (Chapter 2). As in other areas of the Slave, it therefore appears that the peak of regional metamorphism was approximately 2.6 Ga. The HBGB is bordered to the west by plutonic rocks of granodioritic to granitic composition. A granite to the northwest of the HBGB yielded a U-Pb zircon age of 2608 +/- 5 Ma (Bevier and Gebert, 1991), suggesting it correlates with the syn-D2 suite of SSP granitoids (van Breemen et al., 1992). Geochemistry Introduction Samples analysed in this study are dominantly of lower greenschist facies assemblages with rare exceptions that experienced upper greenschist grade metamorphism. Areas of high 58 strain and alteration (predominantly carbonate) were avoided if possible when sampling. Although an attempt was made to sample only primary felsic strata (flows and pyroclastic deposits), a small proportion of epiclastic samples was included. However, epiclastic units invariably contained suspected primary volcanic clasts and where possible the clasts from the epiclastic units were selectively sampled. Most felsic flows and pyroclastic rock samples were feldspar- +\- quartz-phyric. Sampling of the mafic lithologies proved far less problematic with sampling of the pillowed flows restricted to the cores of pillows to minimise the effects of spilitisation. Representative samples collected at surface exposures were in excess of one kilogram for fine-grained rocks and two kilograms for coarse grained or porphyritic rocks. All weathered surfaces were removed prior to processing. One hundred and seventy-eight samples from the HBGB were analysed by X-ray fluorescence (XRF) for ten major and six trace elements (Table 3.1) at Chemex Labs Ltd. in Vancouver, British Columbia. Major elements were determined on fusion discs and trace elements on pressed powder pellets. Nineteen samples from the data set were further analysed for rare earth elements (REE) (Table 3.2) using inductively coupled plasma emission mass spectrometry (ICP-MS) at Activation Laboratories Ltd. in Ancaster, Ontario. Quality control consisted of blind duplicates and in-house standards and documented excellent reproducibility for analyses of major and trace elements when analytical levels were well above the detection limits (Appendix 1). However, the reproducibility of Nb near detection limits (mainly with the XRF data) in two cases was poor, therefore limiting its usefulness for low Nb concentration samples. 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E £ £ - _ -cx a. n. a. a. a. a a a a a a g; E"g "p E E •? £ "? E E a. a. I E £ £ a. £ £ S ft a a a ft a & >UOZONOOc<oo>'NZ2ooooOcajOcuZr>oWOHQa: E tP a is * CQ H D 67 2 ca ft £ £ ^ " ~' in ^' M M ^ M 6 M d N d ^ d d m ro Os ro CN (N C7\ h fN „ SO Tf cs Tf d —«' SO — 00 Tf co ^ co t** in in CN in os so — rsooTf ^ ' 1 . — ~, ^ ^ ^ © od — cd Tf "! co © — so p os m cs _j — m od O(N000so0sC0tNS0C0\OlOln — mSOlOTf r^-rsomrs- — r^rscs _i ^ CN — _' ro cs — rs — t—5 ^-J IT - >o - in d ro ^ rs rs so J£J in © ^ w o d Tf SO m —' od „ P os _; o, . ^ rs ^ • rs, • CN - 0 ^ Od"' O ' ' o — lN. — — oorooomCNmtnroso — so O ro OO _;OsoO^;-^r-.rN-roL-0 i O LO O OS^SCN^ 0nd(Ntrstd^d odd CN 00 so oo CN rs ^osoTfTf^oror^rouo^.— asrocofNsoin'Or^ ' O 1 ro ™ SO CN • (N in CN OO ro SO — ro ^ ro Tf ro — in tn Tf m — _! Tf O f^cSTf™™^^^r^unrNinosoorocsr^ininro^os^^cN ^^^l>OO^d^Os""^^^^^^ro^0d^Srd^dP d ^. O — Tf*. — ro.roo O <N — . O O CSO Tf  O 1 •—1 »n oo Tf 8 - = « NO M r*j • _^ rt tnovi\t^^^m\tn -NO .• O d ri d - -oo m cK CN "! CN CN ci m cN Nt — ri <^ — d m o ^ oo ^ rf „; CN O CN rf d d —' 'p £ s £ ft °. > o o ..EE ft a a E E E E E E E ft a a a a CL ft D. ft a. E E al ft g- & ft a. o. g. ft a ft °" ft ^ ^ 11 ? CL, ft ft ft ft ft •- 3 c ca y-c^^TT-S^ C X f> CO CO CJ Z "2 g 3 TJ X ^ o ZUNOGc«^>-NZ2wcy}OcajUftZwWOHOE ft a. CL E r? E E E E E ft a. a. ft a. ft ft a. ft a. E ' a. BHD | Oi 68 Major and Trace Elements Uncertainties about the nature of plate tectonic and petrochemical processes that operated in the Late Archean hinder detailed interpretation of the lithogeochemistry of greenstone belts. Many workers believe the Archean mantle may have been significantly different from the Phanerozoic and Proterozoic mantle, precluding direct comparison of major and trace element characteristics of Archean rocks with those from various modern tectonic settings. This qualification should be kept in mind for the following discussion, in which geochemical signatures established from the recent rock record will be used to suggest possible paleotectonic settings for rocks from the HBGB. Rocks in ancient greenstone belts have experienced alteration and metasomatism of variable intensity that may modify primary igneous chemical compositions. Therefore, caution must be exercised attempting to interpret first order geochemical characteristics. Large ion lithophile elements (LILE), K, Na, Rb, Sr, and Ba may all be mobile during low-grade metamorphism, therefore little reliance can be placed on them. However, many workers are in general agreement that high field strength elements (HFSE), Al, Y, P, MREE, and HREE are relatively insensitive to the secondary alteration processes in greenstone belts (e.g. Rollinson, 1993; Kerrich and Wyman, 1997). Thus, greater reliance will be placed on the HFSE, Ti, Zr, Y, Al, Nb, and the REE. Although, greenschist facies metamorphic conditions affected the entire HBGB, primary igneous textures such as pillows and flow banding are commonly well preserved, therefore the prefix "meta" has been abandoned and igneous nomenclature is used to describe the HBGB volcanic rocks. Variation diagrams were constructed (Figure 3.4) to test the relative mobility and incompatibility of AI2O3, TiCh, P2O5, and assess the utility of the HFSE as discriminants in identifying and characterising magmatic suites within the HBGB. The near constant ratios of 69 180 140 1 | ioo H 60 { o o 20 1 0 o£ 200 400 Zr (ppm) 3.50 3.00 2.50 2.00 | 1-50 i 1.00 0.50 0.00 • * • • o o • Clover L fin • Koignuk R. fm • Windy L. fin • Square L. fm • Wolverine L fm A Son Volt fm o Flake L fm • Hayden fm 600 200 400 600 Zr (ppm) 3.50 i 3.00 2.50 H I 2.00 | 1.50 1.00 0.50 0.00 4 50 100 Y(ppm) 150 3.50 i i on -J.UU 2.50 -Co I 2.00 -6 P 1.50 -1.00 -0.50 -0.00 -200 0.00 5.00 10.00 15.00 AlA(wt%) 20.00 25.00 Figure 3.3a. Inter-element variation plots. Element pairs exhibiting strong linear relationships are thought to be incompatible in melts and immobile during metasomatism. 70 3.50 3.00 2.50 f 2.00 9 1.50 H 1.00 0.50 0.00 0.00 0.20 0.40 P2Os (wt %) 0.60 0.50 0.40 « 0.30 b" 0.20 0.10 0.00 0.60 50 • Clover L fm • Koignuk R. fm • Windy L. fin » Square L. fm • Wolverine L fm A Son Volt fm o Flake L fm • Hayden fin 100 Y(ppm) 150 200 25.00 20.00 -? 15.00 | o" §f 10.00 5.00-0.00 - 50 o o 100 Y(ppm) 150 0.60 0.50 0.40 1. 0.30 j 6 eC* 0.20 i 0.10 —' 0.00 200 OJ 00 5.00 10.00 15.00 AlA(wt%) 20.00 25.00 Figure 3.3b. Inter-element variation plots. Element pairs exhibiting strong linear relationships are thought to be incompatible in melts and immobile during metasomatism. 71 Ti02, Zr, and Y suggest these elements behaved incompatibly and hence, did not partition into the phenocryst phases in rocks of all compositions. In addition, P2O5 and AI2O3 appear to behave incompatibly in rocks of mafic compositions. It should be noted that non-linear scatter within some element pairs is not only caused by mobility and compatibility but may also arise from: 1) the presence of mixed rock suites, 2) difficulty in sampling true liquid compositions (particularly problematical with calc-alkaline felsic rocks), 3) variable fractionating assemblages during fractional crystallisation, 4) sampling bias and 5) a combination of the above (Rollison, 1993). In the following discussion less reliance will be placed on elements which display random scatter. Chemical classification of rocks from the HBGB Rocks within the HBGB data set span basalt to rhyolite compositions based on a Si02 versus Zr/Ti02 plot (3.5a) after Winchester and Floyd (1977). A Sin2 frequency diagram (Figure 3.5b) indicates a tri-modal rock distribution for the HBGB corresponding to peaks between 48-54 %, 66-70 % and 74-78 % weight percent Si02 Magmatic affinities for supracrustal rocks within the HBGB was determined through immobile and incompatible elements Zr and Y (Figure 3.6) using the guidelines of MacLean and Barret (1993). Mafic (Hayden formation) and intermediate composition rocks (Son Volt formation) are dominantly of tholeiitic and transitional affinities, respectively, with calc-alkaline rocks dominating the main felsic successions in the HBGB (Wolverine Lake, Square Lake, Windy Lake, Koignuk River, and Clover Lake formations). However, a rare spatially- restricted tholeiitic rhyolite unit (Flake Lake formation) is an exception to the felsic trend. 72 a) 40 1 .— .— •— ,— 0.001 0.01 0.1 1 10 Zr/Ti02 35 T 48 52 56 60 64 68 72 76 80 SiQ2 (wt %) Figure 3.5. a) Si02 (wt %) versus Zr/Ti02 discrimination plot after Winchester and Floyd (1977). The plot illustrates the compositional range of the data set from the HBGB. b) Histogram of Si02 frequency distribution for rocks of the HBGB. 73 180 -i 0 100 200 300 400 500 600 Zr (ppm) Figure 3.6. Y-Zr magmatic affinity plot after Barrett and MacLean (1993). Zr/Y values of 2-4.5, 4.5-7, and >7 correspond to tholeiitic, transitional, and calc-alkaline affinities, respectively. 74 Lithogeochemistry Mafic rocks : Mafic rocks are the most abundant preserved volcanic lithology in the HBGB. All mafic volcanic rocks within the HBGB are collectively grouped together, forming the Hayden formation (Figure 3.2 and 3.3). Based on HFS inter-element ratios, primarily Ti02/Zr (Figure 3.7) and REE patterns (Figure 3.8) (Table 3.3) the Hayden formation may contain up to three distinct mafic suites: BG-1, BG-2, and BG-3. The regionally extensive BG-1 suite forms the dominant lithology in the HBGB. BG-1 flows are tholeiites characterised by mean HFSE ratios: TiCVZr = 161, Zr/Y = 3, Ti02/Y = 53, Al203/Ti02 = 14, P205/Zr = 15, P205/Y = 50, Ti02/ P205 = 11, and Mg# of 49. On a chondrite normalised diagram (Figure 3.8) selected (n=3) BG-1 rocks plot as a relatively flat coherent group at around 10 times chondrite with [La/Yb]N = 0.8-1.0, [La/Sm]N = 0.9, [Gd/Yb]N = 0.9-1.0, with a slight negative Eu anomaly (Eu/Eu* = 0.9). Primitive mantle normalised REE (Figure 3.8) are relatively flat with marked troughs in Nb and P, likely resulting from the presence of titanite and apatite, respectively or alternately the troughs suggests a sialic contribution (Barley, 1986) to this suite. The low abundances of elements with high ionic potential (Nb, Zr, Y, Ti) and REE profiles on chondrite and primitive mantled normalised plots are consistent with a MORB, island arc or back-arc setting. The low Zr/Nb ratio (19.5-20.7), relatively flat REE profile, and lack of strongly depleted LREE is more typical of basalts erupted in a back-arc basin setting (Wilson, 1989, Kerrich and Wyman, 1997). Dominantly situated in the northern HBGB in the upper portion of the Hayden formation mafic pile, a rare commonly pillowed mafic suite of tholeiitic to transitional basaltic andesite composition comprises BG-2 rocks. This suite is characterised by relatively higher abundances of HFSE and REE with mean HFSE inter-element ratios of Ti02/Zr = 98, Ti02/Y = 75 3.50 • BG-l o 3.00 • BG-2 • BG-3 o 2.50 -2.00 1.50 1.00 0.50 0.00 0 ON O 100 200 300 Zr (ppm) 400 Figure 3.7. Binary immobile element plot of Ti02 vs. Zr suggesting the presence of three distinct mafic magmatic suites. a) 100 CD 1 O JS u CJ 10 100 § CL) > 10 • BG-1 • BG-2 n BG-3 • • • • $ t t * • * • o 8 • • • La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Element • \ • © • • o g 8 • • •— o X f 1 I * • t : i * Th Nb La Ce P Nd Hf Zr Sm Ti Y Yb Lu Element Figure 3.8. a) Chondrite normalised rare earth element plot for mafic rocks of the Hay den formation. Normalising values from Boynton (1984). b) Primitive mantle normalised rare earth element plot for mafic rocks from the Hayden formation. Normalising values from Sun and McDonough(1989). 77 8Z. O r r ^ g ^ pop ic is oo 3 II O UJ to O to 0 NO UJ 1 I I —! M « Ui j Ui J> O to UJ b\ ~j r* !*" 9s 1 ui « ^ m P r r- 1 NO 00 NO t° 3 W W p y UJ ui o O K'N I i to 3 UJ J> UJ "|| 4^ O ON NJ i i i O O N> — i— J> —• a to P II K'N ON N-> to UJ H. ° H ~J H O NO H-H > ^ ^ 5 "° 2 S 5 o P ^ N 5 o TO O ^ N uO ^ ^ N J> J>- UJ ON NOtOOOUiUiUJoui NO NJ ~jtooN<jJj>K-ooo J> J> NO tO — CO NO oouiooouimoooo NJJ OO NOMWVIA-UU OS to J> to UJ .—' — NO J> 4^tOOJtOUJU)ONO — to to so J> UJ to J> — -o J> NO •— to u, CN I Ul O J> to -O ^4 UJ ON I-4 J^ OS I -O UJ Ui J>. >—. Ul i M H (J NO Ui O to ~J O t> W ON w J> o so — to UJ Ul v] H U M ^1 w i NJ o OS ~J — J> Ui • ON O I C\ J> u * N H Ui -O H- UJ SO UJ to H- Ul to to o\ Ul H- to OS. 00 UJ os SO UJ o NO UJ to J> oo so — to _ i— to to J> i N— ON — UJ UJ ~j J> tO H- to 00 to Ui ON I— I — SO O OS J> UJ 00 I I I I I I I I 3 CO II o ui T -O CO O • to 03 II o to 00 O a 3 II to ^1 et 3 Ii o 3 £1 r II g> 3 3 f J?1 r ?rii (Tl K> 2. o fro 3 3 Q <1V o 1 a < 3 oi &. r § R-49, Zr/Y = 5, Al203/Ti02 =13, P205/Zr = 20, P205/Y =101, Ti02/ P205 = 5 and a Mg# = 48 (Table 3.3). REE patterns (Figure 3.8) for BG-2 are typified by fractionated LREE and HREE over relatively flat MREE ([La/Yb]N = 2.3-5.0, [La/Sm]N = 0.9-1.4, [Gd/Yb]N = 1.9-2.5) and a slight negative Eu anomaly (Eu/Eu* = 0.9). Primitive mantle normalised REE patterns (Figure 3.8) are variably spiked with deep troughs at Th, Nb, P, and Ti and peaks at Ce, Nd, and Sm. REE patterns and trace element signatures are suggestive of an ocean island, E-MORB, or back-arc basin setting for this rock suite (Wilson, 1989, Kerrich and Wyman, 1997). As with the previous rock suite, basalts of BG-3 rarely crop out within the HBGB and could not be distinguished from BG-1 in the field. This Fe-tholeiitic suite is represented by 5 samples and is characterised by HFSE ratios of Ti02/Zr = 249, Zr/Y = 3, Ti02/Y = 69, Al203/Ti02 - 12, P205/Zr = 12, P205/Y = 34, Ti02/ P205 = 20, and a Mg# = 44 (Table 3.3). The REE pattern (Figure 3.8) for a sole representative of this suite displays slight fractionation of the LREE ([La/Yb]N =1.4), and relatively flat MREE and HREE ([La/SmN = 1.1 and [Gd/Yb]N) at approximately 10 times chondrite with a negative Eu anomaly (Eu/Eu* = 0.9). On a primitive mantle normalised (Figure 3.8) REE diagram BG-3 is characterised by a relatively flat profile with major troughs at Nb and P. The trace element and REE geochemistry is consistent with either an E-MORB, back-arc, or island arc setting. The low Zr/Nb ratio (15.7) and slight LREE enrichment are consistent with either a back-arc basin or an E-MORB setting. The similar REE patterns (Figure 3.8), slight overlap of most HFSE ratios (Table 3.3) with BG-1 rocks and the random distribution of this suite among BG-1 flows suggest that this unit is comprised of metasomatized BG-1 rocks rather than a distinct rock suite. Intermediate rocks Although normally considered rare in greenstone belts, andesitic flows and associated pyroclastic units comprise a significant proportion (~ 10-15 %) of exposed supracrustal rocks in 79 the HBGB. All intermediate extrusive and hypabyssal rocks within the HBGB are assigned to the Son Volt formation (Figure 3.2 and Figure 3.3). Son Volt lithologies are dominantly comprised of epiclastic rocks, although massive and pillowed units occur locally. Plagioclase occurs as the primary phenocryst phase with rare subordinate hornblende phenocrysts present. The formation is characterised by mean HFSE ratios of Ti02/Zr = 68, Zr/Y = 7, Ti02/Y = 45, P205/Zr = 14, P205/Y = 90, Ti02/ P205 = 5 (Table 3.3) and is of transitional magmatic affinity. A subset of samples, including an intrusive phase thought to be a subvolcanic feeder to the volcanic suite, display similar REE patterns (Figure 3.9). LREE are enriched over the MREE and HREE, resulting with moderate [La/Yb]N = 4.7-6.7, [La/SmN = 2.6-3.2, and [Gd/Yb]N = 1.3-1.5 ratios. The Eu anomaly (Eu/Eu* = 0.9-1.1) when it occurs is minor and varies from positive to negative in the suite. Primitive mantle normalised REE patterns display pronounced troughs at Nb, P, and Ti and peaks at Th, La-Ce, and Nd-Sm. The deep troughs indicate this suite has experienced crustal contamination (Kerrich and Wyman. 1997; Poliat et al., 1998). The fractionated REE patterns and marked troughs at Th, Nb and Ti are consistent with a magmatic arc setting for this suite (Poliat et al., 1998, Wilson, 1989). Felsic Rocks Felsic volcanic and associated reworked pyroclastic rocks comprise approximately 30 % of the exposed supracrustal rocks in the HBGB. These rocks are divided into six formations (Flake Lake, Wolverine Lake, Square Lake, Windy Lake, Koignuk River, and Clover Lake formations; Figure 3.2 and 3.3) based on field relationships and age constraints. Structural data (pillow tops) suggest the Flake Lake formation locally overlies the volumous mafic pile northwest of Spyder Lake (Figure 3.2b) and age constraints (Chapter 2) indicate that it is the oldest felsic volcanic package in the HBGB. Rocks of this formation occur as massive quartz-eye rhyolitic volcanic flows of tholeiitic affinity characterised by mean HFSE 80 Th Nb La Ce P Nd Hf Zr Sm Ti Y Yb Lu Element Figure 3.9. a) Chondrite normalised rare earth element plot for intermediate rocks from the Son Volt formation. Normalising values from Boynton (1984). b) Primitive mantle normalised rare earth element plot for intermediate rocks from the Son Volt formation. Normalising values from Sun and McDonough (1989). 81 ratios of: Ti02/Zr = 5, Ti02/Y = 2, and Zr/Y = 4, P205/Zr - 1, P205/Y = 3, and Ti02/ P205 = 7 (Table 3.3). Chondrite normalised REE patterns (Figure 3.10) are mildly fractionated ([La/Yb]N =1.9) at around 100 times chondrite, LREE enriched ([La/Gd]N = 1.8), with relatively flat HREE's ([Gd/Yb]N = 0.9 and a pronounced negative Eu anomaly (Eu/Eu* =0.7) consistent with a highly evolved melt. Primitive mantle normalised REE patterns (Figure 3.10) are nearly flat with pronounced troughs at P and Ti and a minor trough at Nb. This type of felsic rock has not previously been recognised in the SSP and is absent in most Archean cratons except in the Superior Province (Thurston and Fryer 1983; Lesher et al., 1986; Barrie et al., 1993; Jackson et al., 1994), where it commonly forms the major host for volcanogenic massive sulphide deposits (Barrie et al., 1993;Thurston 1981; Lesher et al, 1986). The REE patterns, high abundance of HFSE, and inter-element ratios are suggestive of a back-arc basin or MORB setting for this suite (Pearce et al., 1984; Lentz, 1998; Barrie et al., 1993). The Wolverine formation, comprising a series of hypabyssal rhyodacite intrusions, crops out in the northern portion of the HBGB southwest of Patch Lake (Figure 3.2a). This formation is represented by a single sample of calc-alkaline affinity with inter-element ratios of Ti02/Zr = 35, Zr/Y = 22, Ti02/Y = 42, P205/Zr = 9, P205/Y = 200, Ti02/ P205 = 3.81 (Table 3.3). Chondrite normalised REE pattern's (Figure 3.10) are moderately fractionated ([La/Yb] = 20.0, [La/Sm] = 3.5, and [Gd/Yb] = 3.3) with distinct troughs at Nb and P on a primitive mantle normalised REE plot (Figure 3.10). Spatially restricted highly fragmental calc-alkaline felsic suites located in the west central HBGB comprise the Square Lake formation (Figure 3.2). Rocks of this formation are characterised by variable inter-element ratios (Table 3.3) with moderately fractionated chondrite normalised REE patterns ([La/Yb] = 23.5-23.6, [La/Sm] = 3.4-4.0, and [Gd/Yb] = 3.0-3.4; Figure 3.10) and troughs at Nb, P, and Ti on a primitive mantle normalised plot (Figure 3.10). 82 a) •c -a a o 0 o O 1000 100 i A Clover Lake fm • Koignuk River fm • Windy Lake fm O Square Lake fm • Wolverine Lake fm • Flake Lake fm 10 1 * 11 •: • A A o.i La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Element Figure 3.10. a) Chondrite normalised rare earth element plot for felsic rock formations from the HBGB. Normalising values from Boynton (1984). b) Primitive mantle normalised rare earth element plot for felsic rock formations from the HBGB. Normalising values from Sun and McDonough(1989). 83 Felsic volcanic and associated volcaniclastic rocks of the Windy Lake formation span the length of the HBGB (Figure 3.2), and form one of two regionally extensive felsic formations. Element ratios (Table 3.3) suggest this formation is comprised of at least two distinct calc-alkaline magma suites. FG-1 (Table 3.3) rocks are distinguished from FG-2 rocks by mean HFSE ratios of Ti02/Zr = 35 and P205/Zr =15. REE patterns (Figure 3.10) for this suite are strongly fractionated ([La/Yb]N = 71.2) with LREE and HREE enrichment ([La/Sm]N =6.0, and [Gd/Yb]N = 5.0) and a minor negative Eu anomaly (Eu/Eu* = 0.9). Primitive mantle normalised REE patterns display marked embayments at Nb, Ti, and P (Figure 3.10). A strongly altered, spatially restricted pyroclastic unit characterised by constant Ti02/Zr =12 and P205/Zr = 4 comprise FG-2 rocks (Table 3.3). The regionally extensive Koignuk formation includes a series of calc-alkaline felsic units that crop out along the western margin of the HBGB (Figure 3.2). REE patterns (Figure 3.10) are characterised by shallow sloping fractionated REE patterns ([La/YbJN = 10.6-13.9) with LREE enrichment ([La/Sm]N =4.2) and relatively flat HREE ([Gd/Yb]N = 1.7-2.1) and a minor negative Eu anomaly (Eu/Eu* =0.9) on a chondrite normalised plot. Primitive mantle normalised REE patterns (Figure 3.10) display marked troughs at Nb, Ti, and P. Inter-element ratios (Table 3.3) are fairly uniform given the porphyritic nature typical of calc-alkaline rocks and are characterised by mean values of Ti02/Zr = 33, Zr/Y = 12, Al203/Ti02 = 31, P2Os/Zr = 8, Ti02/ P205 = 4. Rocks comprising the Clover Lake formation crop out as a massive quartz-poor calc-alkaline rhyolite flow. The unit is thus far spatially restricted to a sole occurrence in the southern portion of the HBGB (Figure 3.2b). HFSE ratios (Ti02/Zr = 18, Zr/Y = 14, Ti02/Y = 16, P205/Zr = 20, P205/Y = 289, Ti02/ P205 = 1; Table 3.3), highly fractionated REE patterns ([La/Yb]N = 217.1, [La/Sm]N =3.9, and [Gd/Yb]N = 30.0; Figure 3.10), and pronounced negative Eu and Yb distinguish the suite from other felsic suites in the HBGB. A significant negative Eu 84 anomaly (Eu/Eu* = 0.4) is present as well as is a strongly pronounced negative Yb anomaly. The Eu anomaly is believed to result from the weak alteration of feldspars or the result of the magmatic fractionation of plagioclase while the Yb anomaly likely reflects analytical measurement difficulties. Primitive mantle normalised REE patterns (Figure 3.10) demonstrate pronounced troughs at Nb, P, Ti, and Yb. Primitive mantle normalised REE patterns (Figure 3.10) for all calc-alkaline formations are typified by deep embayments at Nb, P, and Ti consistent with melts that have experience crustal contamination. Inter-element ratio's and fractionated REE patterns coupled with Nb, P, and Ti depletions suggest a magmatic arc setting for calc-alkaline rocks in the HBGB (Wilson, 1989, Pearce and Peate, 1995; McCulloch and Gamble, 1991). Discussion and Conclusions Determining an evolutionary model for the HBGB is fraught with difficulties ranging from our poor understanding of Archean systems to the "grungy" and often poorly preserved rock record in the HBGB and other Archean greenstone belts. Any one model must explain the tholeiitic to calc-alkaline progression of the volcanic pile and the presence of two geochemically distinct.mafic suites (BG-1 and BG-2) within the Hayden formation documented in this study. Given the protracted evolution of the HBGB (>116 m.y.) and diverse geochemical signatures of the volcanic sequences, the present stratigraphy of the HBGB is thought to reflect episodic magmatism formed in an arc/back-arc geodynamic setting analogous to the present Mariana or the Tonga-Kermadec arc. The evolution of the HBGB began with the deposition of the >2716 Ma Hayden formation, comprised of the dominant BG-1 suite and the subordinate BG-2 suite. However, BG-1 and BG-2 rocks appear to have evolved in different geodynamic settings. HFSE ratios, flat REE patterns and a lack of strongly depleted LREE patterns for BG-1 rocks is consistent 85 with deposition in a back-arc basin, whereas the enriched LREE patterns of the subordinate BG-2 suite suggest this suite is likely to of evolved in an ocean island or enriched MORB settings. However, geochemical diversity of basalts in a back-arc settings is seen in the Mariana and Lau Basins (Stern et al., 1990; Ewart et al., 1998; and Gribble et al., 1998) where basalts of MORB and ocean island affinities coexist. Often the MORB basalts grade into ocean island basalts if slab roll back has occurred allowing OIB enriched melts to enter the back-arc source region. This appears to be the case with the Hayden formation. BG-2 rocks are crudely restricted to the upper mafic pile of the Hayden formation above the ubiquitous BG-1 suite. Given the position of the BG-2 rocks at the top of the mafic pile below the transitional and calc-alkaline rock formations coupled with the low Nb values for this suite it is likely this suite was also deposited in a back-arc setting. During the waning stages of mafic volcanism the deposition of the tholeiitic 2716 Ma Flake Lake formation occurred. The flat REE pattern, the HFSE abundances, and position near the top of the Hayden formation are consistent with this unit forming in the same back-arc basin as the mafic rocks (BG-1 and BG-2) of the Hayden formation. The Flake formation possibly representing the differentiated product of a high level Hayden mafic magma or the partial melting of mafic or felsic rocks (Barrie et al., 1993). Overlying the Hayden and Flake Lake formation, are intermediate rocks of the Son Volt formation. Fractionated REE patterns and marked Nb, P, and Ti depletions signify an evolution from back-arc spreading to an arc building phase, for the HBGB. This formation is in turn overlaid by a series of temporally distinct felsic formations (2690 Ma Square Lake, 2685 Ma Windy Lake, and 2677 Ma Koignuk River formations). The calc-alkaline affinities of these rocks and similar trace element depletions as rocks from the Son Volt formation are typical of modern arc rocks and signify intense arc development. In addition the temporal distribution of these formations indicates arc magmatism occurred episodically. Finally, the Wilco and Hope 86 Bay formation are thought to record the uplift and erosion of the HBGB and mark the cessation of evolution for the HBGB. 87 References Barley, M.E. (1986). Incompatible-element enrichment in Archean basalts: A consequence of contamination by older sialic crust rather than mantle heterogeneity. Geology v. 14, p 947-950. Barrie, C.T., Ludden, J.N., and Green, T.H. (1993). Geochemistry of volcanic rocks associated with Cu-Zn and Ni-Cu deposits in the Abitibi subprovince. Economic Geology v.88, p 1341-1358. Bevier, M.L., and Gebert, J.S. (1991) U-Pb geochronology of the Hope Bay-Elu Inlet area, Bathurst Block, Northeastern Slave Structural Province, N.W.T. Canadian Journal of Earth Sciences, v 28, p 1925-1930. Bowring, S.A., Housh, T.B., and Isachsen, CR. (1990). The Acasta Gneisses: Remnant of the Earth's early crust. 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Current Research, Part C, Geological Survey of Canada, Paper 90-1C, p 151-165. Lesher, CM., Goodwin, A.M., Campbell, I.H., and Gorton M.P. (1986). Trace element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada. Can. J. Earth Sci. 23, 222-237. Lentz, D.R. (1998). Petrogenic evolution of felsic volcanic sequences associated with Phanerozoic volcanic-hosted massive sulphide systems: the role of extensional geodynamics. Lindsey, D. (1998) Geochemical and Petrographic Study of the Mafic-Ultramafic Suite in the Hope Bay Volcanic Belt, Slave Structural Province, NWT. B.Sc. thesis, University of British Columbia 55 p MacLean, W.H. and Barrett, T.J. (1993). Lithogeochemical techniques using immobile elements. Journal of Geochemical Exploration, v. 48, p 109-133. McCulloch, M.T., and Gamble, J.A. (1991). Geochemical and geodynamical constraints on subduction zone magmatism. Earth and Planetary Science Letters, v. 102, p 358-374. McLennan, S.M. and Taylor, S.R.(1984). Archean sedimentary rocks and their to the composition of the Archean continental crust. In A. Kroner, G. Hanson, and A. Goodwin (eds), Archean Geochemistry, Springer-Verlag, Berlin, p 47-72. Mortensen., J.K., Relf, C, Davis, W.J., and King, J.E. (1992). U-Pb zircon ages from the Shallow Bay volcaniclastic belt, Contwoyto Lake area, NWT: Age constraints for Lupin-type iron formation. Radiogenic Age and Isotope Studies: Report 5, Geological Survey of Canada, Paper 91-2, p 9-15. 89 Padgham, W.A. (1985). Observations and speculations on supracrustal successions in the Slave Structural Province. In L.D. Ayres, P.C Thurston, K.D. Card, and W. Weber (eds), Evolution of Archaean Sequences. Geological Association of Canada. Special Paper 28, p 133-151. Padgham, W.A. (1991). The Slave Province, an overview; Geological Survey of Canada, Open File 2168, pp. 1-40. Pearce, J.A., Harris, N.B.W., and Tindle, A.G. (1984). Trace element discrimination diagrams for tectonic interpretation of granitic rocks. Journal of Petrology, v. 25, p 956-983. Pearce, J.A., and Peate, D.W. (1995). Tectonic implications of the composition of volcanic arc magmas. Annual Review of Earth and Planetary Sciences, v. 23, p 251-285. Poliat, A., Kerrich, R., and Wyman, D.A. (1998). The late Archean Schreiber-Hemlo and White River-Dayohessarah greenstone belts, Superior Province: collages of ocean plateaus, oceanic arcs, and subduction accretion complexes. Tectonphysics, v 289, p 295-326. Rollison, H. (1993). Using geochemical data: evaluation, presentation, interpretation. Longman, Harlow, p 352. Stern, R.J., Lin, P.N., Morris, J.D., Jackson, M.C., Fryer, F., Bloomer, S.H., and Ito, E. (1990). Enriched back-arc basin basalts from the northern Mariana Trough: implications for the magmatic evolution of back-arc basalts. Earth and Planetary Science Letters, v 100, p 210-225. Stern, R. and Bleeker, W. (1997). Geology and SHRIMP zircon geochronology of the Acasta gneisses; initial results. Geological Survey of Canada, Program with Abstracts, p 4. Sun, S.S. and McDonough, W.F. (1989). Chemical and isotopic systematics of oceanic basalts; implications for mantle composition and processes. In Saunders A.D. and Norry M.J. (eds.), Magmatism in Ocean basins. Geolical Society of London, Special Publication, v 42, p313-345. Thorpe, R.I., Cumming, G.I., and Mortensen, J.K. (1992). A significant Pb isotope boundary in the Slave Province and its probable relation to ancient basement in the western Slave Province. In Project summaries: Canada-Northwest Territories Mineral Development Subsidiary Agreement Geological Survey of Canada, Open File 2484, p 179-164. Thurston, P.C. (1981). Economic evaluation of Archean felsic volcanic rocks using REE geochemistry. In Archean geology (ed J.E Glover and D.I. Groves), Special publication No.7, geological Society of Australia, p 439-450. Thurston, P.C. and Fryer, B.J. (1983). The geochemistry of repetitive cyclical volcanism from basalt through rhyolite in the Uchi-Confederation greenstone belt, Canada. Contribut. Mineral. Petrol. 83, 204-226. 90 van Breemen, O., Davis, W.J., and King, J.E. (1992). Temporal distribution of granitoid plutonic rocks in the Slave Province, Northwest Canadian Shield, Canadian Journal of Earth Sciences v.29, p 2186-2199. Wilson, M. (1989). Igneous Petrogenesis: A global tectonic approach. Chapman and Hall, New York, 466p. Winchester, J.A. and Floyd, P.A. (1977). Geochemical discriminants of different magma series and their differentiation producsts using immobile elements. Chemical Geology, v. 20, p 325-343. Yamashita, K, Jensen, J.E., Creaser, R.A., and Gebert, J.S. (1995). Geology, geochemistry, and Nd isotopic study of the Hanikahimajuk Lake area (NTS 86 1/2, 86 H/14,15), northern Point lake belt, Slave Stuctural Province, NWT; Geological Association of Canada, Program with Abstracts, v.20, p A-l 12. 91 Chapter 4 Conclusion Twenty-one U-Pb zircon and titanite ages, one hundred and seventy four major and trace element analyses, and nineteen rare earth element analyses were produced in this study. The U-Pb geochronology together with the lithogeochemistry and geological mapping provides a relatively detailed tectonic evolutionary framework for the formation of supracrustal successions within the Late Archean HBGB. The HBGB is characterised by a basal series of mafic dominated tholeiitic volcanic flows (Young Group) overlain by a sequence of calc-alkaline volcanic rocks (Westerberg Group) that are in turn overlain by sedimentary rocks of the Tweedy and Farrar group. These successions were deposited over a period in excess of 116 m.y. from ca. 2716 to ca. 2600 Ma. Chemical compositions of volcanic rocks are typified by low abundances of HFSE and depletions in Nb, Ti, Eu, and P relative to REE. The striking similarity between the overall lithologic assemblages and the geochemical signature of volcanic rocks in the HBGB with modern arc and back-arc systems (e.g. Mariana and Tonga-Kermadec regions) suggest the HBGB evolved in an arc geodynamic setting. The Young Group, a series of primarily mafic flows, was deposited in a back-arc basin, the bulk of which was deposited before ca. 2716 Ma. Detrital zircon ages (ca. 3.3 and 2.8 Ga) from a turbidite within the HBGB may provide evidence for the existence of ancient crystalline basement in the vicinity. This was followed by deposition of the Westerberg Group, which comprises arc magmatism that occurred between ca. 2699-ca. 2677 Ma. Age constraints suggest that this arc magmatism was episodic. Following the main stage of arc formation the deposition of ca. 2663 Ma Tweedy and ca. 2600 Ma Farrar Group, thought to record the erosion and 92 subsequent uplift of the HBGB, respectively, mark the end of the depositional history of the HBGB. Results from this study raise several questions. The most pressing questions are: 1) Although, the bulk of mafic volcanism in the HBGB is assumed to have occurred prior to ca. 2716 Ma, minor basalt flows are distributed sporadically throughout the stratigraphy. Are there any younger mafic flows, or is the distribution of mafic rocks throughout the stratigraphy a function of structural repetition? Did mafic volcanism occur contemporaneously with arc magmatism? 2) Chronological constraints suggest the presence of thrust faults within the HBGB. Are they Archean structures or the result of the Proterozoic Thelon and/or Wopmay orogen? 3) What age is the Son Volt formation? It is at present loosely bracketed between 2716 Ma (age of the Flake Lake formation) and 2699 Ma (age of the overlying Wolverine Lake formation). The Son Volt formation represents the earliest phase of arc magmatism within the HBGB and an age for this unit would better constrain the total evolution of arc activity in the HBGB. 4) Does crystalline basement exist within the eastern SSP? No basement has been documented in the eastern SSP thus far. However, detrital zircon ages (ca. 2.8 and 3.3 Ga) with little evidence of prolonged transport indicate an ancient basement source. Did these zircon crystals originate from the western SSP where basement is known to exist or were they derived from a basement block within the Bathurst Block or just east, outboard of the present eastern boundary of the SSP? 93 Appendix 1 Analytical Precision Three duplicate Mineral Deposit Research Unit in-house standards and seven field duplicate sample pairs were submitted with batches of samples analysed, to examine the precision of the data (XRF and TCP-MS). XRF Analytical errors (Table 5.1) for major element data were often better than 5 % with rare exceptions exceeding this value. Trace elements exhibited a larger range of values, typical relative errors were below 10 % in most cases except for Nb which displayed larger errors likely in response to values in close proximity to the detection limit. ICP-MS Analytical errors (Table 5.2) for rare earth and transition element data rarely exceed 10 %. Errors associated with the rare earth elements and high field strength elements most commonly used in this study, rarely exceeded 5 %. 94 CQ < CQ II o CL IN CO Q. cn 3 CL CO cn t-CL CO cn 2 Q CL ror CO CO Tf 00 rf CO cn rf CM o o LO CO o o CO CO 00 o LO CO o o O O o o CD IN % Er d d d d CM d d d d rf rf d d CM d rf an o CM CD IN CN CO cp CO 00 CO 00 rf O CM IN- cn CM CD O O o O o q o o o o 00 o Me, LO LO cb d d CM d LO d d LO rf CN CO CN CM o i*-CD ro co CO ror 00 O CN CM CO LO 00 LO O CN o o CO 00 o r» cn cn IN O O f-LO O O o o CO o HI d d d d d d d d CM d CO rf d d CM d CO '['; > i • an CM CM CM IN 00 o LO CN rf LO CP CN CM cn LO O, d'. o •o o o o o o o LO Me; CO CD rf rf CN d rf d d d' CO CD if) CN rf o I--LO CD CN CD ror CM CD CM CO 00 CO 00 00 IN IN CD O CD CO rf O r-co o o O O o rf LO o o % Er d d d d d d CO d CN CM d d d oi d an rr CO CN rf CD O) LO cn rf oo o O O rf CM ro rf O LO o o O q o o o LO o o Me; IN LO IN CO CO rf ^ d LO d d 1-^ l>-CD rf CO CN CO rf CM IN ro cd ror CM CO CO LO CO LO IN ro CN CO CD <J) cn CD CM CD o o LO CN CD CM CM LO CD CM o IN rf o o LU d d CD T ai d IN LO d CD LO oi LO CO CM rf an CM r- o CO CM rf CO o CO CO cn o IN o ro CO rf O CM CO O LO o o O O o o O O o o Me: c\i IN ^ CN LO CN d d d d CM rf CN rf CO CO IN CM d CN rf LO CM ror CD CO cn CN CM rf 00 rf CO o cn CO cn CD cn rf CO CO CO CO rf CO O O ro o ro CM 00 o o %Er d d d CO CN IN CN cd CN LO CN d rf T d an CO CD o rf CN LO CN o CO LO 00 f-o CO CD CN CO rf o LO o o o o o o o LO o o Me CO CD LO CO CO d d rf d d CM CO LO CM IN CN CO rf CN CD ror CD CM O CO f-o CD o LO o o CO O ro CD CO LO ro CO CO CO CN IN iN rf rf % Er d d LO d CM d CM IN d CD CO CO LO LO CM CM an CN CD LO o CO CO rf o CN r-CD co cn CD o LO o LO O LO o o O LO o LO Me CO LO CD rf IN d CO d LO d d CO ro CD CO ro CO CN d CM ror CO CM CO CO CD CN LO CD LO CO CO o o f-co CD CN CO CO CD CO CD rf CO o o OO CO o o % Er d d CO CD d CM d T— LO CO d ro d d CN CO d an 1 CM CM CD f- CD LO CO rf O CO o CM CM o CN CO o LO o q o LO o o o o o o Me, CO CD LO CN CN d d CD d d CN CN CO CO cn CM LO CO o LO JOJ CO o rf l~~ IN 00 CO LO CO CO LO o ro rf ro o o o rf O O LO CO LO CO % Er d d CO d d d LO CM LO d d d LO d CM CO rf an CD cn 00 LO CO LO rf o o rf IN LO cn o CM CD CO CD LO o LO o o O LO o o o o o LO Me, CN CD LO rf LO CM d CO d d ro LO LO CO CM h-ro LO CO IN Element Si02 % AI203 % CaO % oN CO o CM CD LL o CN MgO % MnO % o CM CO 2 cN LO o CN CL Ti02 % CO CO 43 or v_ CO J2 Z N 95 Figure 5.2. Duplicate analyses for ICP-MS data 97PQMW109 96PFMW004 n=2 • i ., n=2 Element Mean % Error Mean % Error V (ppm) 68.50 2.19 120.00 0.83 Cr (ppm) 52.50 4.76 76^.50 1.96 Co (ppm) 10.50 4.76 24.00 0.00 Ni (ppm) 20.50 7.32 80.50 0.62 Cu (ppm) 14.50 3.45 65.00 1.54 Zn (ppm) 53.00 11.32 76.50 1.96 Ga (ppm) 17.00 0.00 17.00 0.00 Ge (ppm) 0.65 7.69 1.10 0.00 Rb (ppm) 74.00 0.00 1.30 23.08 Sr (ppm) 185.00 1.62 263.50 0.95 Y (ppm) 9.75 2.56 15.00 0.00 Zr (ppm) 134.00 2.24 119.00 0.84 Nb (ppm) 6.65 2.26 5.40 0.00 Mo (ppm) 2.80 7.14 0.65 7.69 Sb (ppm) 0.14 3.70 0.55 4.59 Cs (ppm) 1.30 0.00 0.40 0.00 Ba (ppm) 501.50 1.50 22.50 2.22 La (ppm) 24.85 7.44 11.60 2.59 Ce (ppm) 54.30 6.08 24.65 2.23 Pr (ppm) 6.21 5.69 2.82 2.36 Nd (ppm) 23.65 6.98 11.55 2.16 Sm (ppm) 3.82 4.85 2.61 3.26 Eu (ppm) 1.03 5.12 0.81 0.81 Gd (ppm) 2.47 0.20 2.57 1.56 Tb (ppm) 0.33 4.62 0.44 3.45 Dy (ppm) 1.85 4.07 2.60 2.31 Ho (ppm) 0.33 3.03 0.52 1.92 Er (ppm) 1.01 3.96 1.54 1.30 Tm (ppm) 0.14 7.37 0.24 3.28 Yb (ppm) 0.69 3.65 1.44 0.69 Lu (ppm) 0.14 6.47 0.24 2.11 Hf(ppm) 3.65 6.85 3.15 1.59 Ta (ppm) 0.51 1.96 0.66 0.00 W (ppm) 0.75 6.67 5.05 2.97 Tl (ppm) 0.46 4.35 0.02 56.50 Pb (ppm) 0.50 38.75 7.00 0.00 Bi (ppm) -0.05 0.00 0.25 6.12 Th (ppm) 3.01 6.98 1.68 1.19 U (ppm) 0.45 1.12 0.16 6.25 96 

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