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Temporal, geochemical, isotopic, and metallogenic studies of mid-cretaceous magmatism in the Tintina… Heffernan, R. Scott 2004

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TEMPORAL, GEOCHEMICAL, ISOTOPIC, AND METALLOGENIC STUDIES OF MID-CRETACEOUS MAGMATISM IN THE TINTINA GOLD PROVINCE, SOUTHEASTERN YUKON AND SOUTHWESTERN NORTHWEST TERRITORIES, CANADA by R. SCOTT HEFFERNAN B.Sc. Specialization in Geology, University of Alberta - Edmonton, Alberta, 1998 A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF THE REQUIREMENTS FOR THE DEGREE OF M A S T E R OF SCIENCE IN THE F A C U L T Y OF G R A D U A T E STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH C O L U M B I A June 2004 © R. Scott Heffernan, 2004 Library Authorization In presenting this thesis in partial fulfillment 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. I 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. Name of Author (please print) Date (dd/mm/yyyy) Title of Thesis: 7^vt>e^ , Gz&CrtZvMCAL ,Z^Sc>T&PIC Al/ZTsiaLa^O / c Degree: }J\.Sc. Y e a r : 2ooV CASJA^A Department of £A€TH ^ QCZA'JA ^>C/£AJQZ5 The University of British Columbia Vancouver, BC Canada Abstract The Tintina Gold Province (TGP) of east-central Alaska, Yukon Territory, and the southwestern Northwest Territories comprises a very large number of gold (± base metal) deposits and occurrences that are spatially and temporally related to mid-Cretaceous intrusions. Intrusions in the eastern Selwyn Basin, south of MacMillan Pass and east of Frances Lakes, include some of the largest bodies within the TGP and are the focus of this study. Magmatic rocks of the TGP have been divided into individual plutonic suites on the basis of crystallization age, lithology, mineralogy, geochemistry, and spatial distribution, as well as metallogenic association. From -111 Ma to -99 Ma, magmatism is thought to reflect the formation of a southwest-facing continental magmatic arc, represented by the Whitehorse - Coffee Creek suite, and that the coeval Anvil and Cassiar suites formed in a back-arc environment. The younger Tay River, Tungsten and Tombstone plutonic suites successively stepped inboard between 99 Ma to 89 Ma. However, the processes leading to such volumetrically significant magmatism remains poorly understood. Intrusions within the study area range in composition from granite to granodiorite with subordinate diorite and are characteristically calc-alkaline, peraluminous to weakly metaluminous, relatively reduced, and typically contain only biotite as the dominant mafic phase. Sixteen new U-Pb ages, ranging from ~107 Ma to -91 Ma, constrain a temporal framework for plutonism across the region that is consistent with the progressively "inboard younging" pattern of magmatism observed in the northern and western portions of the TGP. - Geochemical (major, trace and rare earth elements) characteristics, together with geochronology indicate that the Anvil , Tay River, Tungsten, and Tombstone plutonic suites as originally defined farther to the northwest do continue southeastward and into the southwestern Northwest Territories. Initial Sr ratios and epsilon Nd values (n=20; age corrected for T = 100 Ma) range from 0.70853 to 0.72243 and -6.0 to -17.5, respectively. Lead isotopic compositions (n=20) show relatively narrow ranges for 2 0 6 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 4 Pb, and 2 0 8 Pb/ 2 0 4 Pb ratios of 19.397 to 19.772, 15.697 to 15.829, and 39.461 to 39.883, respectively. A l l radiogenic isotope systematics indicate that these magmas have interacted extensively with or were derived entirely from continental crust. Several spatial and temporal trends are apparent in the data including an increase in overall REE abundance and sNd values, and a decrease in Srjni t ai, values with decreasing age (broadly moving from west to east). These trends may reflect differences in the nature of the underlying basement, potential magma source(s), and/or the melt producing processes that were involved. i i Lead isotope compositions of feldspars from various intrusions and sulphides from associated precious- and base metal deposits and occurrences define narrow and overlapping ranges indicating that the metals in many of the mineral deposits (and prospects) in the region are mostly derived from the mid-Cretaceous TGP intrusions. i i i Table of Contents Abstract i i Table of Contents iv List of Tables vii List of Figures viii Acknowledgements ix Chapter 1 Introduction Introduction 1 Methodology 3 Field Work and Sampling 3 U-Pb Geochronology 3 Lithogeochemistry 3 Radiogenic Isotopic Studies 4 Presentation 4 References 5 Chapter 2 Temporal, geochemical and isotopic studies of mid-Cretaceous magmatism in the Tintina Gold Province in southeastern Yukon and southwestern Northwest Territories: Constraints on the tectonomagmatic evolution of the northern Cordillera Introduction 6 Geologic Overview 7 Pre-Mesozoic Tectonic History 7 Mesozoic and Cenozoic Deformation 8 Mid-Cretaceous Intrusions 9 U - Pb Geochronology 10 Samples and Methodology 10 Results 11 Discussion 18 Lithogeochemistry 19 Analytical Techniques 19 Results 20 Discussion 21 Isotope Geochemistry 22 Analytical Techniques 22 Sr and Nd Isotope Results 23 Pb Isotope Results • 24 Discussion 24 Nature of Magma Sources 24 Tectonic Setting of Mid-Cretaceous Magmatism 27 Where does the Tintina Gold Province go? 29 Conclusions 30 References 31 Chapter 3 Lead isotope signatures of Tintina Gold Province intrusions and associated mineral deposits from southeastern Yukon and southwestern Northwest Territories: Implications for exploration in the southeastern Tintina Gold Province Introduction 66 Regional Metallogeny 67 Geologic Background 67 v Lead Isotope Study 68 Samples and Analytical Techniques 68 Results 69 Discussion 70 Conclusions 71 References 71 Chapter 4 Summary and Directions for Future Research Summary 78 Directions for Future Research 80 References 81 Appendix 1 Analytical Precision Analytical Precision 82 VI List of Tables Table 2.1 U-Pb sample locations and descriptions 38 Table 2.2 U-Pb analytical data for southeastern Tintina Gold Province intrusions 39 Table 2.3 Geochemistry sample locations and descriptions 42 Table 2.4 Major, trace, and REE analytical data 43 Table 2.5 Pb isotope data 47 Table 2.6 Sr and Nd isotope data 48 Table 3.1 Location and descriptions of intrusion-related deposits and occurrences 73 Table 3.2 Pb isotope data from feldspar and sulphide mineral separates 74 Table A . l Replicate Standard Analyses 83 List of Figures Figure 1 Location of the study area within the Tintina Gold Province 2 Figure 2.1 Map of Cretaceous volcanic and plutonic rocks in Yukon and Alaska 49 Figure 2.2 Morphogeological belts of the Canadian Cordillera 50 Figure 2.3 Location of U-Pb samples 51 Figure 2.4 U-Pb concordia plots for intrusions in the southeastern Tintina Gold province (1 through 16) 52 Figure 2.5 Map of plutonic suites in the southeastern Tintina Gold Province 56 Figure 2.6 Location of lithogeochemistry and radiogenic isotope samples 57 Figure 2.7 Total alkalis versus silica plot 58 Figure 2.8 Geochemical classification plots (cordilleran vs. ferroan, modified alkali lime index, Shand's Index) 59 Figure 2.9 Primitive mantle normalized REE diagrams (A-D) 60 Figure 2.10 Tectonic discriminant plots 61 Figure 2.11 Thorogenic, uranogenic, and 2 0 8 Pb/ 2 0 6 Pb vs. 2 0 7 Pb/ 2 0 6 Pb plots for intrusions in the southeastern Tintina Gold Province 62 Figure 2.12 Initial 8 7 Sr/ 8 6 Sr and sNd data for intrusions in the southeastern Tintina Gold Province 63 Figure 2.13 Compiled Pb isotope data from the northern Cordillera 64 Figure 2.14 Map of Tintina Gold Province with Tintina Fault displacement restored 65 Figure 3.1 Location of the study area within the Tintina Gold Province 75 Figure 3.2 Map of Cretaceous volcanic and plutonic rocks in Yukon and Alaska 76 Figure 3.3 2 0 7 Pb/ 2 0 6 Pb versus 2 0 8 Pb/ 2 0 6 Pb plot of lead isotope ratios from Tintina Gold Province intrusions 77 Acknowledgements Special thanks go to, my supervisor Dr. Jim K. Mortensen for his caffeine-fuelled harangues, Drs. Janet Gabites and Rich Friedman for their expert tutelage, patience, and guidance through the dark world that is geochronology and isotope geochemistry, and Drs. Dick Tosdal and Steve Rowins for their roles on my supervisory committee. Many thanks go to all those that in one way or another shared in this experience, in particular Mr. Jefferson and the gang. Most importantly, to John and Marty Heffernan and the rest of my family I say thank you for your endless support. ix Chapter 1 Introduction The Tintina Gold Province (TGP) is an arcuate belt of Early to Late Cretaceous intrusive rocks and associated gold-rich mineral deposits and occurrences which extends from east-central Alaska into southwestern Northwest Territories (Fig.l; Smith, 2000). In central and western Yukon, discrete plutonic suites have been recognized on the basis of their age, lithological, and geochemical characteristics, as well as their metallogenic associations (e.g., Mortensen et al., 2000). The connection between individual plutonic suites and their metallogenic associations has proven to be valuable as an exploration tool, enabling exploration companies to target specific groups of intrusions. Intrusions in the southeastern Yukon include some of the largest bodies within the TGP; however little is known regarding their ages or geochemical affinities, highlighting the need for a temporal and geochemical framework to guide future exploration in this large region. A field and laboratory based geochronological, geochemical, and isotopic study was undertaken in order to investigate the southeastern extension of well-established plutonic suites currently recognized in central and western Yukon. New data reported in this thesis help constrain the tectonomagmatic evolution of the region in mid-Cretaceous time, and have implications for genetic and exploration models recently developed for gold and base metal mineralization associated with intrusions in this area (e.g., Lang et al., 2000). Funding for this project was provided by several organizations. Analytical costs (including U-Pb geochronology, Sr, Nd, Pb isotopic analyses, and whole rock geochemistry) were covered by a NSERC Discovery Grant (to J.K. Mortensen) and a NSERC Collaborative Research and Development Grant (to J.K. Mortensen and the Mineral Deposit Research Unit at the University of British 1 Columbia). The Yukon Geological Survey also contributed funding for lithogeochemical analyses and provided logistical support in the field. Additional logistical support and funding for fieldwork was provided by Hudson Bay Exploration and Development Co. Ltd. and a Hugh E. McKinstry Grant to the author. Figure 1. Map of Alaska and Yukon Territory showing the extent of the Tintina Gold Province and the locations of gold deposits and occurrences within (after Mortensen et al., 2000). The main study area is outlined by the box. 2 Methodology Field Work and Sampling Field work was carried out by the author during the 1999 and 2000 field seasons while employed by Hudson Bay Exploration and Development Company Ltd., and working in conjunction with geologists from the Yukon Geological Survey. Limited mapping traverses were completed in and adjacent to several of the intrusions and numerous mineral occurrences and deposits were examined and sampled. Samples of fresh plutonic rocks and mineralized wallrocks and intrusives were collected for petrographical, geochronological, geochemical, and isotopic studies. Additional samples were provided by V . Sterenberg (DIAND Yellowknife), which allowed sample coverage to be expanded to include the southeasternmost extension of the TGP. U-Pb Geochronology Previous geochronologic studies, which mainly utilized K-Ar and Rb-Sr dating methods, indicate broadly mid-Cretaceous ages for intrusions in the study area (e.g., Gordey and Anderson, 1993). However these data are too imprecise and sparse to permit direct comparison with recently developed plutonic suite designations elsewhere in the TGP (e.g., Mortensen et al., 2000). Crystallization ages determined by U-Pb dating of zircon, monazite, and titanite was employed in this study to provide a temporal framework for magmatism in the study area and to assess possible extensions of known plutonic suites into the area. A l l U-Pb sample preparation and analyses were carried out by the author and staff of the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia 3 Lithogeochemistry Lithogeochemical analyses including major, trace and rare earth element (REE) concentrations were utilized to characterize the composition of plutonic rocks and to aid interpretation of the paleotectonic setting in which mid-Cretaceous magmatism in the study area occurred. A l l samples were prepared by the author and analyses were done by A L S Chemex Laboratories in North Vancouver, British Columbia, utilizing a combination of X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) techniques. Radiogenic Isotopic Studies Isotopic studies can provide important information concerning the source of magmas, to evaluate contributions from mantle versus crustal sources, and to identify potential metal sources in mineral occurrences. A representative suite of samples of intrusive rocks providing the broadest temporal and geographical coverage was selected for Rb-Sr, Sm-Nd, and Pb-Pb isotopic analysis. Sulphides from skarn occurrences associated with the plutonic rocks were also sampled for common Pb isotopic study to evaluate possible genetic relationships between skarn mineralization and the mid-Cretaceous magmatism. A l l sample preparation was done by the author. Common Pb analyses were carried out by the author and staff of the PCIGR at the University of British Columbia. Rb-Sr and Sm-Nd isotopic analyses of rock powders previously prepared for whole rock geochemistry were conducted by Dr. R. Creaser in the Radiogenic Isotope Facility at the University of Alberta. Presentation The results of this research project are presented as two research papers (Chapters 2 and 3) that will be submitted to international refereed journals. Chapter 2 focuses on the nature of intrusive rocks within the study area, and incorporates field observations as well as 4 geochronological, petrographic, geochemical, and isotopic data obtained during the course of this study. These data are used to differentiate between individual plutonic suites throughout the study area. Discussion focuses on the mid-Cretaceous tectonomagmatic evolution of the area, and specifically the nature of source(s) of magmatic components, and interpretation of the paleotectonic setting through comparison with data from the western and northern portions of the TGP, the southern Canadian Cordillera and global analogues. Chapter 3 combines conclusions regarding the nature of magmatism drawn from Chapter 2 with results of common Pb isotopic investigations of intrusions and associated mineral occurrences in order to constrain genetic and exploration models for intrusion-related mineralization in the study area. Chapter 4 provides a synopsis of conclusions from chapters 2 and 3, outlines outstanding questions raised from this study and suggests directions for potential future research. References Gordey, S.P., and Anderson, R.J., 1993. Evolution of the northern Cordilleran miogeocline, Nahanni map area (1051), Yukon and Northwest Territories: Geological Survey of Canada Memoir 428, 214 p. Lang, R.L., Baker, T., Hart, C.J.R., and Mortensen, J.K. 2000. An exploration model for intrusion-related gold systems. Society of Economic Geologists Newsletter, no. 40-1, pp. 6-15. Mortensen, J.K., Hart, C.J.R., Murphy, D.C., and Heffernan, S. 2000. Temporal evolution of Early and Mid-Cretaceous magmatism in the Tintina Gold Belt. In: The Tintina Gold Belt: Concepts, exploration, and discoveries, British Columbia and Yukon Chamber of Mines Cordilleran Roundup Special Volume 2, Vancouver, British Columbia, pp. 49-57. Smith, M . 2000. The Tintina Gold Belt: An emerging gold district in Alaska and Yukon. In: The Tintina Gold Belt: Concepts, exploration, and discoveries, British Columbia and Yukon Chamber of Mines Cordilleran Roundup Special Volume 2, Vancouver, British Columbia, pp. 1-3. 5 Chapter 2 Temporal, geochemical and isotopic studies of mid-Cretaceous magmatism in the Tintina Gold Province in southeastern Yukon and southwestern Northwest Territories: Constraints on the tectonomagmatic evolution of the northern Cordillera Introduction The Tintina Gold Province (TGP) of east-central Alaska, Yukon Territory, and the southwestern Northwest Territories comprises a very large number of gold (± base metal) deposits and occurrences that are spatially and temporally related to mid-Cretaceous intrusions (Fig. 2.1, inset). Magmatism within the TGP occurred in a variety of tectonic settings and was superimposed on diverse terranes of the northern Cordillera. Recent investigations (e.g., Mortensen et al., 2000) have lead to the subdivision of plutonic rocks in the region into individual plutonic suites on the basis of crystallization age, lithology, mineralogy, geochemistry, and spatial distribution, as well as their metallogenic signature (Fig. 2.1). From -111 Ma to -99 Ma, magmatism is thought to reflect the emplacement of a continental magmatic arc, represented by the Whitehorse - Coffee Creek suite (WCCS), with the coeval Anvil (APS) and Cassiar (CPS) suites forming in a back-arc environment. The younger Tay River (TRPS), Tungsten (WPS) and Tombstone plutonic suites (TPS), successively stepped inboard between 99 Ma to 89 Ma, but it remains unclear how much, i f any, of a subduction-related component is present in these plutonic suites. This study focuses on intrusions that compose the southeastern extent of the TGP, which have received little attention thus far. The individual plutonic suites defined farther to the northwest appear to project into this area; however prior to this study there was insufficient data 6 available to determine whether the suites extend into the study area. A n additional question concerns the reason why magmatism appears to terminate abruptly to the southeast. This paper presents new age, geochemical and isotopic data for intrusions in this region. The data also help to constrain potential source materials and provide insight into the tectonic setting in which these magmas were generated. Geologic Overview Pre-Mesozoic Tectonic History Ancestral North America (Laurentia) was mostly assembled by circa 1.9 Ga. In the region of what is now the northern Canadian Cordillera, the westernmost margin is believed to consist of the enigmatic Nahanni domain and the Fort Simpson arc (see Welford et al., 2001; their Fig. 12). These crustal elements formed during the waning stages of the accretionary Wopmay Orogen and are recognized largely on the basis of distinct aeromagnetic signatures and isotopic correlations between basement drill core from the northern Cordillera and basement gneisses of the southern Cordillera (Villenueve et al, 1991; Ross, 1991). Since the amalgamation of Laurentia, the western margin has been subject to episodic rifting and the deposition of passive margin miogeoclinal successions (Monger and Price, 1979; Gabrielse and Yorath, 1991, Ch.18). In the northern Cordillera, sedimentation took place in four main periods: 1.84-1.71 Ga Wernecke Supergroup, 1.815-1.5 Ga Muskwa assemblage, 1.2-0.78 Mackenzie Mountain Supergroup, and 0.8-0.54 Ga Windermere Supergroup; each representing a variety of depositional environments (Ross et al., 2001; Aitken and McMechan, 1991, Ch.5). The Meso-and Neoproterozoic assemblages are overlain by dominantly Paleozoic clastic and carbonate rocks of the Selwyn Basin and Mackenzie Platform. Recent seismic studies have shown these thick sedimentary successions, plus crystalline basement layers, form a tapering wedge that composes most of the crust (25-30 km) underlying the Foreland and Omineca belts whereas 7 Windermere and younger strata compose only the uppermost ~5km of this section (Cook et al., 1999; Welford et al., 2001, Fig. 12; Snyder et al., 2002, Fig. IB). Although the controls on the distribution of Proterozoic sedimentation remain somewhat uncertain (Gabrielse and Yorath, 1991), Cecile et al. (1997) have argued that the geometry of the latest Precambrian rifting of the western margin of Laurentia that underlies was fundamental in influencing Paleozoic sedimentation and subsequent Mesozoic deformation (discussed in next section). The northeast trending Liard Line (Fig. 2.2) is a zone defined by Paleozoic facies patterns and is interpreted to represent an ancient transfer fault separating an upper plate margin (Macdonald High) on the south from a lower plate margin (Meilleur River Embayment) on the north (Cecile e ta l , 1997). Mesozoic and Cenozoic Deformation Historically, intrusions throughout the region have been collectively referred to as the Selwyn Plutonic Suite (Anderson, 1983, 1987, 1988; Pigage and Anderson, 1985; Gordey and Anderson, 1993). Intrusions of the southeastern TGP were emplaced into Late Precambrian to Mesozoic strata of the Selwyn Basin and define the eastern limit of the Omineca Belt in the northern Cordillera (Fig. 2.2). Selwyn Basin strata consist mostly of turbiditic sandstones, deep water limestones, shale and chert which were deposited contemporaneously with shallow water carbonate rocks and sandstones of the Mackenzie platform to the north and west (Gordey and Anderson, 1993). Jurassic through Paleoene collisional deformation, resulting from the accretion of allochthonous terranes to the west (Monger and Price, 1979; Gabrielse and Yorath, 1991), produced the northeast-verging Selwyn and Mackenzie fold belts. The former is characterized by small- to large-scale, open to tight folds with associated axial planar cleavage whereas the latter formed large scale, broad open folds. The different styles of deformation reflect varying competency of the strata involved; weak, thin-bedded siliciclastic strata of the Selwyn Basin 8 versus the competent thick carbonate sections of the Mackenzie platform (Gordey and Anderson, 1993), respectively. Palinspastic reconstructions suggest crustal shortening of approximately 30% or about 50 to 70 km in this area, which is significantly less than the 200 km of contemporaneous shortening observed in the southern Canadian Cordillera (Gabrielse and Yorath, 1991; Gordey and Anderson, 1993; Cecile et al., 1997). The emplacement of mid-Cretaceous intrusions post-dates regional compressional deformation (folding and thrusting) but is pre- to syn-kinematic with respect to other major faults within the region (Gordey and Anderson, 1993; Murphy, 1997). The only known extrusive equivalents to these plutonic rocks in the region are caldera-filling, welded dacitic tuffs of the South Fork Volcanics (Gordey, 1988), which are considered to be comagmatic with the Tay River suite intrusions (Mortensen et al., 2000). Mid-Cretaceous Intrusions Mid-Cretaceous intrusions in southeastern Yukon and southwestern Northwest Territories form simple to complex, single to multi-phase stocks, plutons and batholiths. Contacts with adjacent wallrocks are typically steep (>65°) and are characterized by prominent oxidized, rusty weathering metamorphic aureoles that are less than a few hundred meters in width. The presence of andalusite-bearing hornfels within contact aureoles and known stratigraphic thicknesses indicate a possible range of 3.3 to 11.6 km for depth of emplacement (Gordey and Anderson, 1993). A limited amount of aluminum-in-hornblende thermobarometric data for some of the intrusions in the region (Heffernan, unpub. data) also suggest shallow emplacement levels, and possibly further constrain emplacement depths to less than ~6 km. Major intrusive phases consist mainly of medium-grained, massive to megacrystic granodiorite, quartz monzonite, and granite. Porphyritic (plagioclase ± hornblende ± biotite) phases and aplite dykes are locally abundant, particularly near the margins. Biotite and 9 magnetite ± ilmenite are the dominant ferromagnesian phases. Hornblende is noticeably absent except within the Coal River Batholith. Common accessory minerals include apatite, zircon, allanite, and monazite or titanite. See Table 2.3 for a complete list of sample lithologies. Weak to moderate alteration of hornblende (if present) and biotite to chlorite and epidote, and sericitization of plagioclase are ubiquitous, but generally weak. U - P b Geochronology Samples and Methodology U-Pb geochronology was used to constrain crystallization ages of intrusions within the study area. Samples were collected from surface exposures of individual intrusions or from different phases of composite intrusions to provide the broadest geographical coverage throughout the region. Table 2.1 provides a summary of samples, including the name of the intrusive body sampled, U T M coordinates and a brief lithologic description of samples for which crystallization ages are being reported. Zircon, monazite and/or titanite was separated from 3-15 kilogram samples using conventional crushing, grinding, and Wilfley table techniques, followed by heavy liquid and magnetic separation steps. Mineral fractions for analysis were selected based on grain size, quality, morphology, and magnetic susceptibility. Most zircon fractions were abraded prior to dissolution using the technique of Krogh (1982) in order to minimize the effects of post-crystallization lead loss. A small number of zircon fractions (see table 2.2) were not abraded; nor were all monazite and titanite fractions. A l l geochemical separations and mass spectrometry were done in the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. Samples were dissolved in concentrated HF and HNO3 in the presence of a mixed Z - " ~ Z J 3 U - u ; ipb tracer. Separation and purification of Pb and U employed ion exchange column techniques modified slightly from those of Parrish et al. (1987). Pb and U 10 were eluted separately and loaded together on a single Re filament using a phosphoric acid silica-gel emitter. Isotopic ratios were measured using a modified single collector VG-54R thermal ionization mass spectrometer equipped with a Daly photomultiplier. Most measurements were done in peak switching mode on the Daly detector. U and Pb analytical blanks were in the range of 1-2 pg and 1-9 pg, respectively, during the course of this study. U fractionation was determined directly on individual runs using a mixed 2 3 3 " 2 3 5 t j tracer, and Pb isotopic ratios were corrected for a fractionation of 0.43%/amu, based on replicate analyses of the NBS-981 Pb standard and the values recommended by Thirwall (2000). A l l analytical errors were propagated through the age calculations using the numerical technique of Roddick (1987). A l l errors are reported at the 2a level and are included with analytical results in Table 2.2. Results Sixteen of the thirty-one samples processed for U-Pb dating returned well-constrained ages. Nine samples did not yield minerals amenable for analyses and an additional six samples require further work to clearly resolve an age. Numbered sample locations (Fig. 2.3) and concordia plots (Fig. 2.4) correspond to samples as discussed below. 1. Mt. Billings Batholith (99-SH-022): This sample is a medium-grained biotite granodiorite that was collected from the southern portion of the Mt. Billings Batholith (Fig. 2.3). A moderate amount of fair to good quality zircon was recovered. Two fractions (A and B) of clear, colorless elongate prisms with minor inclusions and one fraction (C) of clear, colorless stubby equant prisms were moderately abraded and analyzed. A l l three fractions were discordant (Fig. 2.4). A regression line through all three points (MSWD=0) has a lower intercept age of 106.4 ± 0.4 Ma and is interpreted as the igneous crystallization age. The calculated upper intercept age of 1.87 11 Ga indicates a mainly Mesoproterozoic average age for an inherited zircon component contained within the sample. 2. Mt. Billings Batholith (00-SH-001): This sample is a weakly foliated, fine- to medium-grained biotite granodiorite. The sample was collected from surface, close to the margin of the northern end of Mt. Billings Batholith (Fig. 2.3). Very little zircon was recovered from the sample. Most zircon grains were either clear to pale brown, with 'cloudy' inherited cores or were brown to dark brown and contained a very high abundance of inclusions. The sample also yielded a quantity of euhedral clear yellow monazite grains containing very minor bubble-shaped inclusions. Two fractions of monazite, composed of 2 to 3 tabular grains with wedge-shaped terminations, were analyzed. Fractions M4 and M6 overlap each other and plot above concordia (Fig. 2.4), due to excess 2 0 6 Pb from the decay of 2 3 0 T h (Parrish, 1990). Assuming no post-crystallization Pb loss, a conservative crystallization age of 103.5 +/- 1.1 Ma is assigned based 207 235 on the range of Pb/ U ages of the two monazite fractions. 3. Mt. Billings Batholith (SH-073): This sample is a massive, fine-grained, biotite granite collected from surface in the vicinity of the Tia skarn occurrence (Minfile # 105H 073) at the northern end of the Mt. Billings Batholith (Fig. 2.3). The sample contained a small amount of very fine (<100 um) monazite and a very small amount of zircon. Monazite grains were clear pale yellow, had no visible zoning, were virtually inclusion free, and had a subhedral flat/tabular morphology. Two fractions (M4 and M6) were analyzed and yielded similar results (Fig. 2.4). A crystallization age of 100.8 +/- 2.3 Ma is assigned based on the range of Pb/ U ages. 4. Mt. Billings Batholith (SH-070): This sample of fine- to medium-grained granite was collected from the Cali skarn occurrence (Minfile # 105H 070) located at the northern end of the 12 Mt. Billings Batholith (Fig. 2.3). The sample yielded a small amount of moderate to good quality monazite and a very small amount of poor quality zircon. Three fractions of monazite were analyzed. Monazite fractions M5 and M6 each consisted of several grains of clear, pale yellow, broken, stubby prismatic grains. These fractions yield nearly concordant analyses with 2 0 7 P b / 2 3 5 U ages of 104.6 ± 0.5 Ma and 104.4 ± 0.8 Ma respectively. Fraction M4 was a single grain of the same morphology of fractions M5 and M6 except that it was of poorer clarity due to a higher content of micro-inclusions. Fraction M4 yielded a slightly older 2 0 7 P b / 2 3 5 U age of 106.3 ±1.6 Ma. A conservative crystallization age of 105.8 ± 2.2 Ma is assigned based on the Pb/ U age range of all three fractions. 5. Coal River Batholith (99-SH-008): This sample is a medium-grained hornblende biotite granodiorite collected from the central portion of Coal River Batholith (Fig. 2.3). Abundant clear, colorless, prismatic zircon grains of varying morphology were recovered. Most of the zircon were quite fine (<104 urn) and of excellent quality. Three moderate- to strongly-abraded fractions of the coarsest grains were analyzed. Fractions B and C (tabular morphology) yielded different but concordant results and fraction D (elongate morphology) was slightly discordant (Fig. 2.4). With no evidence of inheritance having affected the sample and assuming that fractions C and D have been affected by post-crystallization Pb loss (2-3 times higher U concentration than fraction B), a crystallization age of 95.6 ± 0.4 Ma is assigned based on the Pb/ U age of concordant fraction B. 6. Coal River Batholith (99-SH-006): This sample is a medium-grained hornblende biotite granodiorite collected from the central portion of the batholith, southwest of sample 99-SH-008 (Fig. 2.3). A large quantity of moderate to excellent quality, colorless to pale yellow zircon with variable morphologies was recovered. Three strongly abraded fractions of zircon, each of a 13 different morphology, were analyzed (Fig. 2.4). Most zircon grains in all fractions contained very minor colorless to pale brown rod- and bubble-shaped inclusions. Fractions A and B are concordant with 2 0 6 P b / 2 3 8 U ages of 97.1 ± 0.2 Ma and 96.9 ± 0.3 Ma, respectively. Based on these two fractions, a crystallization age of 96.9 ± 0.4 Ma is assigned. Fraction C was also concordant, but with a slightly younger 2 0 6 P b / 2 3 8 U age (95.4 ± 0.2 Ma). This fraction is interpreted to have suffered minor post-crystallization Pb loss. 7. Caesar Lakes pluton (99-SH-013): This sample of fine-grained biotite granite was collected from near the southern end of the pluton (Fig. 2.3). A moderate amount of variable quality zircon and a small amount of monazite was recovered. Three fractions of unabraded zircon and one fraction of monazite were analyzed. Zircon fractions F and H , which consisted of square prisms with abundant colorless to dark bubble-shaped inclusions and rod-shaped inclusions parallel to the c-axis of the zircons, yielded discordant results (Fig. 2.4). Fraction I also contained abundant inclusions but zircons grains were very elongate with length to width ratios of 5 to 15 and was only slightly discordant. The one fraction of monazite (M4) consisted of three yellow, inclusion free, euhedral grains and plots above concordia with a Pb/ U age of 97.1 ± 0.5 Ma. The three zircon fractions plot slightly below concordia and yield non-overlapping Pb/Pb ages, indicating the presence of small amounts of inherited zircon in each fraction. The monazite age (97.1 ± 0.5 Ma) is taken as the best estimate for a crystallization age of this sample. 8. Tuna stock (99-SH-016): This sample of fine- to medium-grained biotite monzogranite was collected in the vicinity of the porphyry style Tuna mineral occurrence (Minfile # 105H 082)(Fig. 2.3). Abundant monazite of good quality was recovered. Four fractions of the 14 coarsest, clearest euhedral grains were analyzed. A l l fractions (M3 - M6) plot together on or slightly above concordia (Fig. 2.4). A crystallization age of 97.1 ± 2.0 Ma is assigned based on the total range o f 2 0 7 P b / 2 3 5 U ages from all results. 9. Mulhollandpluton (98-HAS-06): This sample is a hornblende biotite granodiorite containing K-feldspar megacrysts up to 3cm. The sample was collected on surface from the Mulholland pluton located in the northeast portion of the study area (Fig. 2.3). Abundant clear, colorless to pale brown, stubby to elongate prismatic zircon was recovered. Three fractions were analyzed (Fig. 2.4). Fraction A consisted of strongly abraded, coarse (> 134 urn) elongate prisms that contained abundant fractures and rod-shaped inclusions parallel to the c-axis of the zircons. A crystallization age of 97.7 ± 0.2 Ma is assigned based on the concordant analyses for this fraction. Fraction C was also composed of elongate prisms but was not abraded. This fraction was discordant and likely reflects post-crystallization Pb loss. Fraction B was discordant and is interpreted to contain an inherited, older component as 'cryptic' cores that could not be distinguished visually. 10. Rudi pluton (98-Z-C-028): This sample is a medium-grained biotite monzogranite collected on surface from the northernmost intrusion sampled in this study (Fig. 2.3). A small amount of good quality monazite was recovered. Three fractions of the coarsest grains, with the fewest bubble-shaped inclusions, and analyzed. Fractions M2, M3, and M4 yielded very similar, concordant results (Fig. 2.4). A crystallization age of 98.0 ± 1 . 2 Ma is given based on the 2 0 7 P b / 2 3 5 U ages of all three fractions. 11. Patterson stock (98-HAS-12): This sample is a fine-grained biotite granodiorite from the south-easternmost intrusion in the study area (Fig. 2.3). The sample was collected from one of 15 many small, scattered exposures of this body. A small amount of zircon of poor to moderate quality was recovered. Four fractions of different morphologies were moderately abraded and analyzed. Fraction B (elongate prisms) and fraction D (multi-faceted equant prisms) were discordant, and although no cores were observed, the fractions are interpreted to have contained an older, inherited components (Fig. 2.4). Fraction A consisted of several colorless, clear, inclusion-free flat tabular grains and plots just off concordia. Fraction C was composed of coarse fragments of elongate prisms and contained abundant fractures and bubble- and rod-shaped inclusions that occurred proximal to or parallel to the c-axis of the zircons. Fraction C is concordant and a crystallization age of 97.5 ± 0.3 Ma is assigned for the sample based on the 2 0 6 P b / 2 3 8 U age of this fraction. 12. Felsic dyke, near Patterson stock (98-HAS-12a): This sample is a biotite-plagioclase porphyritic dyke collected close to the previous sample (Fig. 2.3). A moderate amount of good quality monazite was recovered and two fractions were analyzed. Both fractions consisted of several yellow, clear, euhedral (tabular) monazite grains that were virtually inclusion free. The 707 7TS total range o f z u ' P b / Z J J U ages for the fractions is 98.3 ±1 .6 Ma (Fig. 2.4), which is interpreted as the crystallization age of the dyke. 13. Shannon Creek pluton (99-SH-001): This sample is a weakly foliated, medium-grained biotite granodiorite (Fig. 2.3). Abundant zircon of variable quality and morphology was recovered, and five strongly abraded fractions were analyzed. Fraction B was concordant with a 7 0 A 7 1 8 Pb/ U age of 97.0 ± 0.2 Ma which is interpreted as the crystallization age. The remaining fractions D A , DB, C, and A A returned discordant results reflecting variable amounts of Pb loss and inheritance. 16 14. Mt. Appier pluton (98-HAS-02): This sample is a medium-grained biotite monzogranite and was collected at the northern end of the field area (Fig. 2.3). Moderately abundant colorless to pale brown, stubby to elongate prismatic zircon was recovered. Two moderately abraded fractions (D and F) and one unabraded fraction (E) were analyzed and all returned discordant results (Fig. 2.4). Abundant good quality monazite was also recovered. Five fractions of the best quality grains were analyzed. Fractions M2 and M10 overlap concordia and the remaining fractions ( M l , M7, and M8) plot slightly above concordia indicating the presence of excess 2 0 6 Pb (Fig. 907 9 ^ ^ 2.4). The z u ' P b r J J U ages of all fractions are within error of each other. A crystallization age of 9 07 9 ^  ^ 94.5 ± 0.9 Ma is reported based on the weighted average of all Pb/ U ages. 15. McLeodpluton (98-HAS-14): This sample is a medium-grained biotite monzogranite with K-feldspar phenocrysts up to 15 mm. It was collected from one of the most easterly intrusions in the study area (Fig. 2.3). A large quantity of colorless to pale brown, stubby to elongate prismatic zircon of variable quality was recovered. Three fractions of zircon were strongly abraded and analyzed. Two fractions (A and B) consisted of elongate grains with inclusions and fractures parallel to the c-axis. The finer fraction (B) was concordant and fraction A was slightly discordant (Fig. 2.4). Fraction C was composed of several clear, colorless tabular grains and also returned slightly discordant results. A crystallization age of 93.9 ± 0.2 Ma is assigned based on the Z U 0 P b / Z J 6 U age of the concordant zircon fraction B. Minor angular fragments of clear, pale to medium honey brown titanite were recovered from a more magnetic separate. One unabraded 906 9TR titanite fraction (Tl) yields a concordant (albeit imprecise) analysis with a Pb/ U age of 92.8 ± 0.2 Ma. The slightly younger age for the titanite likely reflects minor post-crystallization Pb loss. 17 16. Big Charlie pluton (99-SH-011): This sample is a fine- to medium-grained biotite monzogranite with K-feldspar megacrysts up to 2.5 cm. The sample was collected from the southeastern region of the study area (Fig. 2.3). Abundant zircon of variable quality was recovered and four fractions were strongly abraded and analyzed. Fraction A A was composed of colorless, elongate, square prisms and yielded a concordant analysis (Fig. 2.4) with a crystallization age of 91.0 ± 0.3 Ma. This is interpreted to give the crystallization age of the sample. The remaining fractions A B , B, and C A were discordant, reflecting an inherited component in each fraction. Discussion U-Pb results presented here provide a temporal, framework for mid-Cretaceous magmatism throughout the study area. The same 'inboard younging' pattern of magmatism recognized in the northern and western portion of the TGP (Mortensen et al., 2000) is evident in the study area. Four samples from the easternmost Mt. Billings Batholith returned the oldest ages, ranging from -107 to -100 Ma, and are correlated with the Anvil plutonic suite (Fig. 2.5). Samples from intrusions to the north and east of the Mt. Billings Batholith, including the Coal River Batholith, the Tuna Stock, and the Mulholland and Rudi plutons, all return ages <100 Ma. The majority of ages are between -99 and -95 Ma, the range of ages that characterizes the coeval Tay River and Tungsten plutonic suites as recognized further north. None of the samples dated in this study are strongly peraluminous or contain both biotite and muscovite, which are characteristics that typify Tungsten suite intrusions. These intrusions are therefore considered part of the Tay River suite (Fig. 2.5). No Tungsten suite intrusions have been identified south of the Cantung mine area thus far. Among the easternmost intrusions, samples from the Mt. Appier, McLeod, and Big Charlie plutons yielded ages 94.5 ± 0.9 Ma, 93.9 ± 0.2 Ma, and 91.0 ± 0.3 Ma respectively (Fig. 18 2.5). The relatively young ages correlate with the Tombstone plutonic suite to the northwest (-94 to -89 Ma) and indicate that the Tombstone plutonic suite, or at least Tombstone age equivalent magmatism, extends into this portion of the Selwyn Basin. Lithogeochemistry Major, trace and rare earth element (REE) geochemistry was employed to characterize the composition of mid-Cretaceous plutonic rocks in the study area and to aid in the interpretation of the paleotectonic setting in which the magmatism occurred. Analytical Techniques A total of thirty-seven samples were analyzed for their major, trace, and rare earth element (REE) concentrations. Samples were collected from surface exposures at locations shown in Figure 2.6. U T M coordinates, the name of the intrusive body sampled, plutonic suite associations, and brief lithologic descriptions are provided in Table 2.3. To avoid the effects of surficial processes, weathered surfaces were removed using a rock saw. Samples were then crushed and powdered using a standard jaw crusher and a tungsten carbide ring mill at A L S Chemex Labs Ltd. in North Vancouver, British Columbia. A l l major, trace and REE analyses were done at A L S Chemex Labs Ltd. in North Vancouver, British Columbia. X-ray fluorescence (XRF) was used for determining major element concentrations and inductively coupled plasma emission mass spectrometry (ICP-MS) methods were used to determine trace and REE concentrations. Replicate analyses of two in-house whole rock standards were used to evaluate analytical precision and accuracy (see Appendix 1). The results, as well as pertinent major and trace element ratios are presented in Table 2.4. 19 Results Intrusive rock units in the study area show a limited lithological and mineralogical range, and have been subdivided into specific plutonic suites based largely on the results of geochronology. The results of major element analyses indicate that the intrusions are dominantly granitic and granodioritic in composition with subordinate diorite (Fig. 2.7). Diorite occurs as porphyritic satellite dykes and areally restricted marginal phases and does not form volumetrically significant intrusive phases. The high-K calc-alkaline intrusions have mixed I-and S-type characteristics (Irvine and Baragar, 1971; Le Maitre et al., 1989). Using the classification scheme proposed by Frost et al. (2001; Figure 2.8), with the exception of a few samples with the highest Si02 concentrations (Fig. 2.8 A), samples from all three plutonic suites in the study area plot as 'magnesian' or Cordilleran (versus 'ferroan' or A-type) and range from calc-alkalic to alkali-calcic (Fig. 2.8 B). The Anvil Plutonic suite (APS) and Tay River Plutonic suite (TRPS) intrusions are weakly to strongly peraluminous whereas the younger Tombstone Plutonic suite (TPS) intrusions cluster on or very near the metaluminous-peraluminous boundary (Fig. 2.8 C). Major element mobility is considered negligible with the exception of several weakly altered and mineralized samples (e.g. SH-005, SH-008, SH-024, and SH-028a) that account for nearly all scatter observed within the data. Primitive mantle normalized REE plots (Fig. 2.9 A-C) are virtually identical for all three plutonic suites, displaying steep profiles with negative Nb, Eu, and Ti anomalies (Table 2.4). APS intrusions have LaN/YbN values from 4.25 to 18.70 and Eu/Eu* values from 0.05 to 0.84, TRPS intrusions have LaN/YbN values from 5.47 to 29.17 and Eu/Eu* values from 0.29 to 0.96, and TPS intrusions have LaN/YbN values from 13.00 to 44.18 and Eu/Eu* values from 0.48 to 0.75. Although there is significant overlap between these ranges, the TPS intrusions show more pronounced LREE-enrichment and less prominent Eu anomalies (Fig. 2.9-D). The observed increase in overall REE abundance with decreasing age (APS > TRPS > TPS) also correlates 20 with a general increase from -700° to ~800°C in calculated zircon saturation temperatures (Table 2.4; Watson and Harrison, 1983). Discussion The LREE-enrichment (high LaN/YbN values) and negative Nb, Ti and Eu anomalies (Table 2.4 and Fig. 2.9) are characteristics typically ascribed to I-type volcanic arc or subduction-related granitoids (Whalen et al., 1994; Jenner, 1996; Christiansen and Keith, 1996; Morris et al., 2000). More recently however, this same "subduction signature" has been attributed to the partial melting of material previously formed in subduction settings, such as immature sedimentary rocks derived from a continental magmatic arc (e.g., Keskin et al.,1998; Selby et al., 1999; Morris et al., 2000). In such cases, it has been argued that the "subduction signature" is actually inherited from the source rocks that were partially melted. Immobile trace and REE concentrations have been commonly used to discriminate between tectonic settings for granitic rocks. The results from this study are plotted on Rb versus Yb+Ta, Ta versus Yb, Nb versus Y , and Rb versus Y+Nb discriminant plots of Pearce et al. (1984), as well as the H f - Rb/10 - Ta*3 and H f - Rb/30 - Ta*3 discriminant plots of Harris et al. (1986) (Figs. 2.10 A through F). The intrusions display a shared affinity between the 'volcanic arc', 'syn-collisional', and 'within-plate' fields on the discriminant plots of Pearce et al. (1984), plotting on or very near the boundaries between these fields (Fig. 2.10 A to D). On the tectonic discriminant plots of Harris et al. (1986), the intrusions plot entirely within the 'within-plate' and 'late/post-collisional' fields (Fig. 2.10 E and F). Despite these ambiguities, the results appear to indicate that these intrusions did not form in a volcanic arc setting and that they are most likely related to a collisional within-plate tectonic setting. This 'non-diagnostic' feature is common to all plutonic suites present in the field area, and has been observed previously by Lang (2000) for TPS intrusions elsewhere within the TGP, as well as by Logan 21 (2001) for mid-Cretaceous granitoids in the southern Canadian Cordillera that were also emplaced into the Foreland and Omineca belts. Isotope Geochemistry Strontium, neodymium, and lead isotopic studies of the intrusive suites were undertaken to further characterize their composition and possible origin. These data provide insight on the nature, age, and composition of source materials and therefore help constrain the tectonic setting(s) in which this magmatism occurred. Analytical Techniques A subset of 20 samples was selected for Sr, Nd, and Pb isotopic study. The location of these samples are denoted with stars in Figure 2.6 and described in Table 2.3. Samples were chosen in order to provide the broadest geographical and temporal coverage of the study area. Rb-Sr and Sm-Nd isotopic analyses were done at the Radiogenic Isotope Facility at the University of Alberta following the procedures of Creaser et al. (1997) and Holmden et al. (1997). Whole rock powders (prepared by A L S Chemex) were analysed for Rb, Sr, Sm, Nd isotopic compositions and concentrations using isotopic dilution mass spectrometry. Analytical results and errors at the 2a level are reported in Table 2.5. Pb isotope geochemistry, including sample preparation, geochemical separations and isotopic measurements were done at the PCIGR at the University of British Columbia. Clean feldspar mineral separates were handpicked, ground and sieved to obtain a 100 - 200 mesh size fraction. The clean plagioclase separates were first leached in dilute HC1, then in a mixture of dilute Hf and HBr, and subsequently dissolved in concentrated Hf. Separation and purification of Pb employed ion exchange column techniques. Samples were converted to bromide, the solution was passed through ion exchange columns in HBr and the lead was eluted in 6N HC1. 22 The total procedural blank on the trace lead chemistry was 100-110 pg. Approximately 10-25 ng of the lead in chloride form was loaded onto a rhenium filament using a phosphoric acid-silica gel emitter, and isotopic compositions were determined on a Faraday collector in peak-switching mode using a modified VG54R thermal ionization mass spectrometer. The measured ratios were corrected for instrumental mass fractionation of 0.12% per mass unit based on repeated measurements of the N.B.S. S R M 981 Standard Isotopic Reference Material and the values recommended by Thirwall (2000). Mass fractionation and analytical errors were numerically propagated throughout all calculations and are presented at the 2a level with results in Table 2.6. Sr and Nd Isotope Results an Q/r Initial o , Sr/ 0 O Sr and epsilon Nd (sNd) values were corrected for an age of 100 Ma and range from 0.70853 to 0.72243 and from -6.0 to -17.5 respectively (Table 2.5). Sr and Nd results are shown plotted against silica content (Fig. 2.11 A and B) and against each other (Fig. 2.11 C). A l l of the intrusions have initial 8 7 Sr/ 8 6 Sr and sNd values that indicate dominantly crustal source rocks for these magmas. Nd and Sr values show a progressive shift across the study area. The lowest sNd values (~ -17.0) and corresponding high initial 8 7 Sr/ 8 6 Sr (~ 0.720) occur in the northwest portion of the field area. Moving to the southeast, the isotopic compositions shift toward the 'least-evolved' values, with the highest eNd values (~ -6.0) and corresponding low initial 8 7 Sr/ 8 6 Sr (~ 0.708). Depleted mantle model ages (TDM ) after Goldstein et al. (1984) range from 1.36 to 2.72 Ga with an average of 1.77 Ga (Table 2.5). Fifteen out of the seventeen samples yield Mesoproterozoic ages between 1.36 and 1.96 Ga. The remaining two samples (SH-011E from the Mt. Billings Batholith and 98-Z-C028 from the Rudi Pluton) have significantly higher 1 4 7 Sm/ 1 4 4 Nd ratios and produce Archean model ages of 2.65 and 2.72 Ga, respectively. 23 Pb Isotope Results A l l three plutonic suites have very similar and highly radiogenic Pb isotope compositions. Collectively, the 2 0 6 Pb/ 2 0 4 Pb ratios range from 19.397 to 19.772, the 2 0 7 Pb/ 2 0 4 Pb ratios range from 15.697 to 15.829, the 2 0 8 Pb/ 2 0 4 Pb ratios range from 39.461 to 39.883, the 2 0 7 p b / 2 0 6 p b m t i o s r a n g e f r o m 0.79962 to 0.81076, and the 2 0 8 Pb/ 2 0 6 Pb ratios range from 2.01221 to 2.03658. The results are plotted with reference to the upper-crustal Pb evolution model (Shale Curve) of Godwin and Sinclair (1982) (Fig. 2.12 A - C). The Shale Curve is of particular relevance to this study as it is based on the Pb isotope compositions of shale-hosted Zn-Pb deposits located within the Canadian Cordillera miogeocline and is interpreted to reflect the Pb isotopic evolution of average upper crustal Pb within the miogeocline. Discussion The observed spatial trends in isotopic data mirror the results of Lang (2000), who studied mid-Cretaceous plutons to the northwest of the study area. The remarkable constancy of major and trace element data suggest that isotopic variation does not reflect differences in the proportions of source components but instead reflect distinct isotopic differences of the underlying continental crust (Pitcher, 1997; Lang, 2000). The location of the strongly peraluminous two-mica WPS (Fig. 2.5) coincides with the most evolved isotopic signatures. Nature of Magma Sources The high initial 8 7Sr/ 8 6Sr, low sNd, relatively radiogenic Pb isotopic compositions, Mesoproterozoic to Archean T D M ages, and the common presence of inherited zircon of Precambrian ages indicate a dominantly crustal source for these magmas. The wide variation in Sr and Nd values with SiC"2 content excludes the possibility that these magmas formed from a 24 single common source over time (Figs. 2.11 A and B). The curvilinear array on the sNd versus 87 86 initial Sr/ Sr plot (Fig. 2.11 C) is characteristic of magmas generated by the mixing of two compositionally and isotopically distinct magmas and/or the assimilation of ancient crustal material by mantle-derived magmas (DePaolo et al., 1992). The absence of field evidence for magma mixing or mingling is supportive of the latter, however, no samples with primitive isotopic signatures were identified during the course of this study, nor have they been previously documented in the study area or elsewhere in this region. This information points toward the possibility of an entirely crustal derivation for these intrusions; however, the involvement of at least a minor isotopically juvenile or mantle-derived contribution cannot be completely ruled out. The Sm-Nd characteristics of rocks that both host and underlie southeastern TGP intrusions have been documented from a variety of sedimentary provenance studies (Ross et al., 1997; Creaser and Erdmer, 1997; Garzione et al., 1997; Ross et al., 2001) and from limited dating and isotopic studies of the Paleoproterozoic crystalline rocks of the Fort Simpson arc (Villeneuve and Theriault, 1991). In light of these data, a significant contribution of lower crustal rocks (e.g., Muskwa Assemblage, Ross 1999 and Nahanni Terrane, Villeneuve and Theriault, 1991), with average sNdioo values of ~ -25, cannot account for the higher eNd values (-10 to -6) observed in the plutonic suites without contribution of a significant isotopically primitive component. Mafic volcanic rocks do occur within the Proterozoic stratigraphy of the Selwyn Basin but are volumetrically minor (at least at the present level of exposure) and therefore are unlikely to have formed a significant contribution to these magmas. This suggests one of two possibilities: first, that the intrusions are mainly of middle to upper crustal origin, where the range of sNd values more closely resembles those of the intrusions; or secondly, that mantle-derived magmas were extensively contaminated by a component of crustal material and have either not yet been identified or did not reach the present level of exposure. 25 Unlike the wide range of Nd and Sr compositions, lead isotopic compositions of the plutons in the study area show relatively minor variation (Fig. 2.12), suggesting that these magmas were derived from a homogenous Pb isotopic reservoir, or from multiple reservoirs that were well homogenized over time. The elevated 2 0 7 Pb/ 2 0 4 Pb ratios are indicative of rocks of old upper crustal origin (Tosdal, 1999) with high time-integrated U/Pb ratios that are similar to, and higher than those used in the Shale Curve model (p. = 12.16). Feldspar lead isotopic compositions from mid-Cretaceous intrusions measured in this study are among the most radiogenic signatures recognized from mid-Cretaceous intrusions in the northern Cordillera thus far (Fig. 2.13). In general, these data are consistent with the Pb in these magmas having been derived entirely from partial melting of model upper crustal rocks or from mantle derived melts that have been extensively contaminated by, and well homogenized with significant upper crustal material. Crustal residence ages (TDM ) provide a qualitative approach to constraining the age(s) of source materials for magmas. The positive correlation between T D M and eNd suggests that differences may be in part related to the ages of source materials. The dominantly Mesoproterozoic model ages (1.36 to 1.96 Ga) overlap with those reported from sedimentary rocks of the northern Cordillera miogeocline, and in particular, Paleozoic rocks of the Selwyn Basin (Garzione et al., 1997). The two Archean model ages (2.65 and 2.72 Ga) are from intrusions on the western side of the study area and more closely resemble the model ages of Proterozoic miogeoclinal sedimentary rocks (Garzione et al., 1997; Ross et al., 2001) and inferred crystalline basement (?) rocks of the Nahanni Terrane (Villeneuve and Theriault, 1991). The presence of abundant inherited and/or xenocrystic zircon further substantiates the derivation of these magmas from crustal or crustally contaminated sources. Several samples (e.g., 99SH-023, 98-HAS-02, 98-HAS-12) consistently yield calculated upper intercept ages between ~1.8 and ~1.9 Ga (see earlier discussion). Another sample (99SH-001) gives an older, 26 albeit poorly constrained, upper intercept age of 2.12 Ga. This range of ages is consistent with ca. 1850 Ma magmatism in the contemporaneous Fort Simpson and Great Bear magmatic arcs (Hoffman and Bowring, 1984; Villenueve and Theriault, 1991; Cook et al., 1999). However provenance studies report detrital zircons of this age in miogeoclinal sedimentary rocks throughout the entire Canadian Cordillera, from Mesoproterozoic (Ross et al., 2001) through to Mesozoic (Boghossian et al., 1996) times. As such, the -1.8 to -1.9 Ga inherited zircon component offers little intra-crustal constraint on the source of isotopically evolved materials in these magmas other than to confirm its presence. Tectonic Setting of mid-Cretaceous Magmatism The spatial and geochemical characteristics of the -112 to -99 Ma plutonic suites are consistent with magmatism in a southwest facing magmatic arc and back-arc region, respectively represented by the WCCS and the APS and CPS suites (Mortensen et al., 2000). Assessment of the tectonic setting within which younger (-99 to -89 Ma) TRPS, WPS, and TPS magmatism occurred is more problematic. The contribution, i f any, from a subduction-related source is unclear because the geochemical and isotopic systematics are consistent with a largely crustal origin (Mortensen et al., 2000; Driver et al., 2000; Lang, 2000). Common characteristics of subduction related granitic batholiths include the association with coeval mafic magmatism (Ducea, 1992) and the presence of primitive isotopic signatures, even in areas underlain by extremely thick crust (Ducea, 1992; Rollinson, 1993). A l l of these characteristics are noticeably absent in the plutons in the study area, although mafic and even ultramafic phases are recognized locally within the TPS further to the northwest (e.g., Lang, 2000) . This suggests that subduction may not have played a significant role in magma genesis in this region. Furthermore, the reconstruction of Plafker and Berg (1994) suggests that an arc-trench gap between -500 and -700 km would have existed in mid-Cretaceous time. This would 27 have required an extremely low angle of subduction (between -12° and ~8°) for which there is little evidence. Indeed the trace element characteristics (LILE- and LREE-enrichment, and negative Nb, Ti , and Eu anomalies) are the only indication that these magmas might be subduction-related (Hildreth and Moorbath, 1988; Hawkesworth et al., 1993; Christiansen and Keith, 1996). The possibility that the subduction-related trace element signature was inherited remains to be tested. Inherent to all models of crustally derived magmatism is a mechanism by which significant heat is introduced into the crust to induce partial melting (Pitcher, 1997). Given the current knowledge of the crustal structure in the northern Cordillera and our understanding of mid-Cretaceous magmatism, three plausible scenarios can be envisaged for the initiation of crustal anatexis in the area. The first possibility is that the intrusions formed from decompression or dehydration melting of overthickened crust depressed into the mantle, implying a purely crustal origin (Patino Douce et al., 1990; Thompson, 1999; Patino Douce, 1999), and indeed the region has been subject to significant crustal thickening in early and middle Mesozoic time (see earlier discussion). There are however volumetric concerns with this model, specifically, could dehydration melting alone account for the nearly 5000 km of granitic rocks within the study area? The other two possibilities involve the upwelling or underplating of hot asthenospheric mantle due to either crustal extension (Pitcher, 1997; Thompson, 1999) or. crust/mantle delamination (Bird, 1979; Kay and Kay, 1993; Houseman and Molnar, 2001). Cretaceous and younger extension is well documented in some parts of the northern Cordillera (Tempelman-Kluit et al., 1991; Murphy, 1997; de Keijzer and Williams, 1999) and Snyder et al. (2002) suggest a mid- to lower crustal detachment i f the high crustal temperatures observed today 28 (Lewis et al., 2002) were present during orogenic evolution. Progressive delamination from west to east could account for the younging of magmatism from west to the east. Similar studies of mid-Cretaceous magmatism elsewhere within the TGP (Selby et al., 1999; Driver et al., 2000; Lang et al., 2000) and of granitoids emplaced within the Omineca Belt in the southern Cordillera (Brandon and Lambert, 1993; Brandon and Smith, 1994; Logan, 2001) all invoke or suggest that these intrusions were entirely derived from the continental crust due to the partial melting of overthickened crust. However none of these studies can completely preclude the involvement of a subduction-related component. Where does the Tintina Gold Province go? With the ~425 km of dextral displacement along the Tintina Fault restored (see Fig. 2.14), plutonic suites of the TGP are seemingly truncated to the south of the study area. Two equally plausible scenarios that may account for the apparent truncation are presented here but remain to be fully tested. The first explanation is that the plutonic suites continue to the south in a manner similar to that outlined herein (i.e., linear and non-overlapping suites) and are simply not exposed at current levels of erosion. Mineral deposits such as the Sa Dena Hes Zn-Pb-Ag skarn, and replacement deposits such as Quartz Lake (Pb, Zn, Ag) and Hyland (Au, Ba) are known to be associated with small volumes of felsic intrusives. These mineral deposits are all thought to be genetically related to buried intrusions of presumably mid-Cretaceous age although none have been dated directly. Indirect dating of mineralization from the past-producing Sa Dena Hes skarn indicate a mid-Cretaceous age (see Chapter 3). Geophysical evidence (e.g., regional aeromag surveys) for large volumes of buried intrusions to the south of the study area is uncertain and requires further investigation. 29 Alternatively, it is possible that the plutonic suites coalesce, swinging to the southwest around the basement promontory roughly demarcated by the location of the Liard Line (Fig. 2.14). If so, this scenario further emphasizes the strong control that the geometry of the latest Proterozoic rifting had on the distribution of Paleozoic sedimentation, Mesozoic deformation and suggests direct links to the genesis of TGP magmatism. Geochronologic, geochemical and isotopic data from the region is insufficient to permit comparison with currently exposed intrusions that may represent the southward continuation of the TGP. Conclusions The primary goal of this study was to investigate the southeastern extent of plutonic suites within the TGP. The combination of geochronological and geochemical data indicates that the known plutonic suites of the western and northern portions of the TGP do continue into southeastern Yukon and southwestern Northwest Territories. Within the study area, APS magmatism is for the most part limited to the westernmost Mt. Billings Batholith. The majority of intrusions, including the large Coal River and Hole-in-the-Wall batholiths, are considered part of the TRPS. The strongly peraluminous two-mica WPS does not appear to extend south of the Cantung area. The WPS suite seems restricted to a region that shows the most evolved isotopic signatures suggesting a.direct correlation with the nature of the underlying basement rocks. The economically significant TPS continues to the extreme southeast, including the Big Charlie and McLeod plutons, appearing to occur discontinuously along the eastern limits of the belt. Without contemporaneous mafic volcanism, it is difficult to conclusively identify the paleotectonic setting of mid-Cretaceous magmatism. However the data presented here provide some insight into the nature of the tectonic setting. Any tectonic model developed for the region must account for the following salient features: 1) trace element signatures consistent with volcanic arc or subduction-related magmatism; 2) the decidedly crustal isotopic signatures, 30 including the highly variable Sr and Nd, and restricted Pb compositions; and 3) the progressive decrease in age from west to east that is now recognized over an incredible strike length (>1000 km). References Anderson, R.G. 1983. Selwyn plutonic suite and its relationship to tungsten mineralization, southeastern Yukon and District of Mackenzie. 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O _ . -1 I I £ ro" § S O £ tz o 8.15 -a =8 S-x: ro to 3 t= «i= tu i s N CO _ U) cu 8..E E Q CO CO P OF-> TO CZ E o 33 CD o N o CO •— c s-to o XI 0. 2 § 22 to 1-o o CL O E x i CN " =) or S IN to u ba: CO Ll_ CZ to 0 O c CL P ositio isoto Comi CL E Ian x i 0 Ian 0. 0 XI 0 cz 0 a. 0 F isoto ased comi E T 3 X> T 3 . 0 an SjS an 0 CO ltrations in analy k Pb, U, ofthefr cu XI £Z tu 0 0. CO O) tz XI ro 0 CO mor 0 XI Q. (O ri- E cu nk 0 rrect Pb ro rrect O XI 0 J r 0 O CN 41 Table 2.3. Locations and descriptions for geochemical samples. In t rus ive B o d y P lu ton ic Sui te No r th ing* Eas t ing* Desc r ip t i on 99-SH-022 Mt. Billings Batholith Anvil 6757200 507300 00-SH-001 Mt. Billings Batholith Anvil 6852852 499996 00-SH-002 Mt. Billings Batholith Anvil 6853027 499680 00-SH-003 Mt. Billings Batholith Anvil 6853027 499680 SH-005 Mt. Billings Batholith Anvil 6778227 517998 SH-008 Mt. Billings Batholith Anvil 6792026 525509 SH-011E Mt. Billings Batholith Anvil 6792830 515480 SH-024 Mt. Billings Batholith Anvil 6789456 532946 SH-028a Mt. Billings Batholith Anvil 6803467 533040 SH-028b Mt. Billings Batholith Anvil 6801832 531635 SH-029 Mt. Billings Batholith Anvil 6809731 529105 SH-070 Mt. Billings Batholith Anvil 6844586 518400 99-SH-001 Shannon Creek pluton Tay River 6860975 511410 99-SH-002 Shannon Creek pluton Tay River 6861110 510575 99-SH-006 Coal River Batholith Tay River 6796900 569700 99-SH-007 Coal River Batholith Tay River 6798125 571000 99-SH-008 Coal River Batholith Tay River 6803635 574000 99-SH-009 Coal River Batholith Tay River 6824250 581700 98-HAS-02 Mt. Appier Pluton Tay River 6900544 552576 98-HAS-03 Faille pluton Tay River 6896228 569674 98-HAS-06 Mulholland (Cirque) pluton Tay River 6886772 570080 98-Z-C028 Rudi Pluton Tay River 6912514 525354 98-HAS-07 Jorgensen pluton Tay River 6751093 645902 98-HAS-12 Patterson pluton Tay River 6751425 631699 98-HAS-12a Patterson pluton Tay River 6751425 631699 98-HAS-12b Patterson pluton Tay River 6751425 631699 98-Z-12 Powers pluton Tay River 6743796 659038 99-SH-013 Caesar Lakes pluton Tay River 6799200 559500 99-SH-014 Caesar Lakes pluton Tay River 6802500 556750 99-SH-015 Caesar Lakes pluton Tay River 6800680 555870 99-SH-016 Tuna stock Tay River 6855058 541644 99-SH-010 Big Charlie pluton Tombstone 6799292 628757 99-SH-011 Big Charlie pluton Tombstone 6789175 624400 99-SH-012 Big Charlie pluton Tombstone 6794800 621528 98-HAS-15 Big Charlie pluton Tombstone 6799598 627564 98-HAS-14 McLeod pluton Tombstone 6775409 632194 98-HAS-14a McLeod pluton Tombstone 6775409 632196 massive med-gr Bt granodiorite massive med-gr Bt granodiorite weakly-foliated med-gr Bt granite fine-gr Bt-PI dioritic enclave altered med-gr Bt granodiorite med-gr Bt granodiorite massive fine-gr Bt granite altered Pl-Bt porphyry (dioritic) altered Pl-Bt porphyry (dioritic) altered fine-gr diorite meg-gr Bt granodiorite altered fine-gr Bt granite weakly foliated med-gr Bt granite weakly foliated med-gr Bt granite massive med-gr Hbl-Bt granodiorite massive med-gr Bt granodiorite massive med-gr Bt granodiorite massive med-gr Bt granodiorite massive med-gr Bt granite megacrystic Bt granodiorite megacrystic Hbl-Bt granodiorite megacrystic Bt granite Hbl-PI-Bt porphyry (dioritic) massive Bt granodiorite Pl-Bt porphyritic dyke (granitic) aplite dyke (granitic) Pl-Hbl-Bt porphyry (dioritic) massive fine-gr Bt granite massive med-gr Bt granodiorite PI phyric diorite dyke megacrystic Bt granite megacrystic Bt granite megacrystic Bt granite coarse-gr Bt granite megacrystic Bt granodiorite megacrystic Bt granite megacrystic Bt monzonite * NAD 27, Zone 9 Abbreviat ions used in descriptions: fine-gr = fine-grained, Mineral abbreviations: Bt = biotite, PI = plagioclase, Hbl = med-gr = medium grained hornblende o D>| 1-CO o - a o a . 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O O <n "O a CO "co o E CD x: o o CD O CD CD o o I*— o co a. CD SI £ £ 5 E CO 13 CD o O & .co T3 j = CD CQ -4—' O CO ° a5 CO > co £ > > — 1 CO CD c x: CO 48 49 Figure 2.2. Map of the Canadian Cordillera showing the location of the study area with reference to morphogeological belts, the outline of the Selwyn Basin-Kechika Trough depositional system (dashed line), and the location of the Liard Line (after Wheeler and McFeely, 1991;Gabrielse et al., 1991; and Cecile et al., 1997). 50 Figure 2.3. Map of study area showing distribution of early and mid-Cretaceous intrusions (shaded areas) and the location of U-Pb samples (circled numbers) as discussed in text. Dotted lines represent major roads. Map modified from Gordey and Makepeace, 1999. 51 B 0 52 55 J30°W J28°W 126°W British Columbia Figure 2.5. Map of study area showing distribution of early and mid-Cretaceous intrusions (light shaded areas). Dashed lines separate designated plutonic suites as discussed in text and the dark shaded area outlines the extent of the overlapping Tungsten suite. Dotted lines represent major roads. Map modified from Gordey and Makepeace, 1999. 56 Figure 2.6. Map of study area showing distribution of early and mid-Cretaceous intrusions (shaded areas) and the location of geochem samples: circles denote wholerock analyses and stars denote wholerock plus Sr, Nd, and Pb analyses. Dotted lines represent major roads. Map modified from Gordey and Makepeace, 1999. 57 1 6 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i Q I I I I I I I I I I I I I I I 3 5 4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5 Si0 2 Figure 2.7. Total alkalis versus silica plot (Le Bas et al., 1986) showing range of rock types sampled during the course of this study. Symbols: • - Anvil suite intrusions, O - Tay River suite intrusions, A - Tombstone suite intrusions. 58 •<* O O E O - O ^ + O ^ N > OX) C3 3^ + § .a O o III o O f l O U JJ ' S o u s ° a O M 3 3 < C 3 o C cu 1 C U 4 2 to ON ft 25 +-> o c "+-"u In O a > < 2 w CO tU s ra a ^ U IS! ^ 'r/"i eg *S " d C3 S >-> ^ 3 tu CU X - ° 1 - H ^ T 3 i—1 .2 g < ' u © T3 x co <U s .s 1.1 O C it C O e .2 CO 3 C U i2 c u o S-< C U CL, 00 • -f s | ctj <u id P CO 6 0 JSP ca <u fa "o j£ o ra * • * CU o to w CD o « 11 s e <3\ oo o O o o u a tu cS cd X C U e CO JZ\ tu on ig 06|/\l+09d/09d 1000 •£ 100 ~ \ — i — i — i — F — r 10 J* u 1 o 1 Pi .01 _i i i i i i i i i i i i_ 1000 « 100 a Th Nb La Ce Nd Zr Sm Eu Gd Ti Dy Y Er Yb Lu 10 £ 1 4*! ° 1 3 .01 i 1 1 1 1 1 1 1 r i i i i i i i i i i i i i i i Th Nb La Ce Nd Zr Sm Eu Gd Ti Dy Y Er Yb Lu 1000 100 t 10 ° 1 p i .01 n i i i i i i i I I i I I r I 1 I I I I I I L _ I I I I I I Th Nb La Ce Nd Zr Sm Eu Gd Ti Dy Y Er Yb Lu 1000 a 100 ccj > u O 10 1 b-.1 b-.01 -| 1 1—i 1 1 r _i i i i i i i i i_ Th Nb La Ce Nd Zr Sm Eu Gd Ti Dy Y Er Yb Lu Figure 2.9. Primitive mantle-normallized rare earth element plots of (A) Anvil plutonic suite samples (n=12); (B) Tay River plutonic suite samples (n=19); (C) Tombstone plutonic suite samples (n=6); and (D) TayRiver suite samples with the compositional fields of the Anvil suite samples (dark shaded) and the Tombstone suite samples (light-shaded) in the background for comparison. Primitve mantle values from Sun and McDonough (1989). 60 Hf Ta*3 Hf Ta*3 Figure 2.10. (A) Rb-Y, (B) Ta-Yb, (C) Nb-Y, and (D) Rb-Y+Nb discrimination diagrams of Pearce et al. (1984). (E) and (F) Hf-Rb-Ta discrimination diagrams of Harris et al. (1986). Symbols as in Figure 2.7. 61 1 1 1 1 1 1 _ 1 / 0 ) \J uj An / / - o / u 1 i i i i i o CO Is-LO CM Is-o CM I s -LO Is-O oo co CM CD O CM s o o S-H on MO oo o c/3 r~- oo O LO o o o Is-3 ° 001 P N 3 g a g 0 ) w eu S i I l i s ^ > "In S cd C, co ^ .a £ O i_ CCi T-I •a H j « , cci s-« < ^ , — i . C3 ccj ^ H : ^ -a -S cu o oo fc« - 3 3 "5 £ M O 5 5=1 o _ o H I 53 « c3 g £ > a o u r « S 3 6 — o CO CU 3 CO el S tU 8 O g CO ^= O S - i s .2 H £ .-a o a .S 0 0 . 0 g 00 00 g CL, •3 -f3 cl) JO oo cu cd "3 to 3 . O CO P G ^ .5 co > oo S O tL) ,—i 2 > a C/3 CD O O 00 Cvt a 00 crj a %-» 3 a Ct-H o C/} 4) 00 00 a 00 o o n I s-o\SP «.a cu ccj O O I4 a 3 co o LO o LO o LO CO CM CM o Is- Is- Is- Is- Is- Is-ci o' O O d d m 001 p ^ 3 o r--d o co CN o CN CO o c\i 2 0 8 p b / 2 0 6 p b CD o CN "Cf o CN in o CN l-N. o CN CO o CN o CN 0_ CD o CM J2 D_ o CM 00 o CN OO CO CO oo m C O 1 a i 1 1 1 1 1 << o V - \ -9 \<v ~ ertainty V Y - c 3 \ itive V o a Represent V ellipse \ o w -\ \ o c3 ° .2 o O T3 g cd cd cu ^ O PQ w ^ • + J „ o .2 & o s 2 - ° 43 PH H S o CU (U o CO a O cu cu CN o S cu cu *-i cr„ > O M ^ fa 9 a T 3 O co cd cd CO cd 2 > S Jo fa § 3 a 5 3 ra CO CO +2 2 3 1 S3 8 cd «-c ft *-T cu d-> co O ra a a . S cu B g CO • 6 B o 1/3 cfcl S o co O S C/3 O > CU S I co T3 . 3 2 -o V °r g S3 « ' a u C N O O O N T3 fi CO cd o i> CU 1 o O H o ^ C * 63 39.8 39 .6 £ 39 .4 2 39 .2 D_ so 3 39 .0 38.8 38 .6 15.85 15.80 15.75 D_ ° 15.70 \-o 3 " 15 .65 p % 15.60 |-15 .55 A /Representative uncertainty ellipse (this study) B Representative uncertainty ellipse (this study) A Tombstone Suite O Tay River Suite • Anvil Suite O Joyce, 2002 CZj) Mortensen, unpub. data c^Selby etal., 1999 ® > Driver et al., 2000 ^ Lang, 2000 O 18.4 18.6 18.8 19.0 19.2 19.4 19.6 2 0 6 p b / 2 0 4 p b 19.8 Figure 2.13. (A) Thorogenic and (B) uranogenic plots of feldspar lead isotope compositions from this study plotted against similar data from across the TGP. Shale Curve of Godwin and Sinclair (1982) is shown for reference. Data from Joyce (2002) - Dawson Range Batholith and Moosehorn Range intrusions; Selby et al. (1999) - Dawson Range Batholith; Mortensen unpub. data - Clear Creek area intrusions; Lang et al. (2000) - TPS intrusions; and Driver et al. (2000) - Cassiar Batholith. 64 Figure 2.14. Map of study the area showing the distribution of TGP magmatism in mid-Cretaceous time with the -425 km of dextral strike-slip displacement along the Tintina Fault restored. Dotted lines represent major roads. Map modified from Gordey and Makepeace, 1999. 65 Chapter 3 Lead isotope signatures of Tintina Gold Province intrusions and associated mineral deposits from southeastern Yukon and southwestern Northwest Territories: Implications for exploration in the southeastern Tintina Gold Province Introduction The Tintina Gold Province (TGP) in east-central Alaska, Yukon Territory, and southwestern Northwest Territories is host to numerous styles of precious- and base-metal mineralization thought to be genetically associated with widespread Early to Late Cretaceous magmatism (Fig. 3.1). Styles of mineralization in the TGP are highly variable and include sheeted quartz-feldspar veins, polymetallic replacement bodies, auriferous breccias, disseminated ores, and Au-rich skarns, as well as epithermal vein systems (especially associated with late Cretaceous intrusive and volcanic rocks). In the early 1990's, the discovery of gold deposits such as Fort Knox and Brewery Creek spurred exploration activity, leading to further discoveries at Pogo, Donlin Creek, True North, Dublin Gulch, Ryan Lode, and Scheelite Dome. The rush of exploration success was accompanied by significant research directed at the development genetic and exploration models. The resultant Intrusion-Related Gold Systems (IRGS) model has been continuously evolving since its inception (Thompson et al., 1999; Lang et al., 2000; Hart et al, 2000; and Mineralium Deposita Vol . 36, No. 6). This paper presents new Pb isotope data from intrusive rocks and several mineral deposits and occurrences from the southeastern portion of the TGP. These data provide insight on metal sources within these systems and hence help to constrain possible relationships between magmatism and mineralization and resulting exploration models. 66 Regional Metallogeny A variety of intrusion-hosted and (probably) intrusion-related deposits and occurrences are known in the eastern Selwyn Basin, including W (± base metal) skarns such as Mactung, Cantung, and Lened, Ag-rich base metal skarns and mantos such as Sa Dena Hes, gold-bearing sheeted quartz-feldspar veins (e.g., within the Mactung intrusion), distal, apparently structurally controlled deposits such as Hyland, and massive sulphide replacement deposits such as Quartz Lake (Macmillan) (Fig. 3.2). Indeed, at least 45% of the 325 MINFILE occurrences listed for the six map sheets that comprise the study area (105-A, -H, -I, and 95-D, -E, -L) are definitely or arguably intrusion-related. A discussion of the mineral potential of the eastern Selwyn Basin would not be complete without mention of the numerous SEDEX-type occurrences such as the Howards Pass deposit and the Matt Berry prospect. The combination of both syngenetic and epigenetic exploration targets has led to a considerable amount of interest from exploration companies in the mineral potential of this region for several decades. Geologic Background One of the most striking features of the southeastern portion of the TGP is the shear volume of granitic magmatism in the area. At current levels of exposure there is over 5000 km of mid-Cretaceous intrusive rocks exposed within the study area. These intrusions comprise simple to complex, single to multi-phase stocks, plutons and batholiths (Fig. 3.2). Intrusions of the southeastern TGP were emplaced into Late Precambrian to Mesozoic strata of the Selwyn Basin. Stratigraphy of the Selwyn Basin consists mostly of turbiditic sandstones, deep water limestones, shale and chert which were deposited contemporaneously with shallow water carbonates and sandstones of the Mackenzie platform to the north and west (Gordey and Anderson, 1993). Results of geochronological, geochemical and isotopic investigations described in Chapter 2 indicate that the known plutonic suites of the western and northern 67 portions of the TGP (Mortensen et al., 2000) do continue into southeastern Yukon and southwestern Northwest Territories. For further discussion on the regional geology see Chapter 2. Lead Isotope Study Lead isotope studies of feldspars separated from various mid-Cretaceous intrusions in the study area, and sulphides from a number of precious- and base metal deposits and occurrences have been carried out in order to investigate possible relationships between mineralization and magmatism. Samples and Analytical Techniques Sulphide samples were collected from numerous mineral deposits and occurrences that were examined during the course of this study. The names and brief descriptions of the sampld occurrences are presented in Table 3.1, and sample locations are shown in Figure 3.2. A l l sample preparation, geochemical separations, and isotopic measurements were done at Pacific Centre for Geochemical and Isotopic Research (PCIGR) at the University of British Columbia. A detailed discussion of samples and analytical techniques for feldspar mineral separates is presented in Chapter 2. For trace lead sulphide samples, approximately 10-50 milligrams of hand picked sulphides minerals were first leached in dilute hydrochloric acid to remove surface contamination and then dissolved in dilute nitric acid. Samples of galena required no leaching and were directly dissolved in dilute hydrochloric acid. Following ion exchange chemistry, approximately 10-25 nanograms of lead in chloride form was loaded on rhenium filaments using a phosphoric acid-silica gel emitter. Isotopic ratios were determined with a modified VG54R thermal ionization mass spectrometer in peak-switching mode on a Faraday detector. Measured ratios were corrected for instrumental mass fractionation of 0.12%/amu based repeated 68 measurements of the NBS 981 standard and the values recommended by Thirwall (2000). Errors were numerically propagated throughout all calculations and are reported at the 2a level (Table 3.2). Results Lead isotopic analyses of feldspar and sulphide mineral separates are presented in Table 3.2 and are plotted with reference to the upper-crustal Pb evolution model (Shale Curve) of Godwin and Sinclair (1982) in Figure 3.3. The Shale Curve is of particular relevance to this study as it is based on the Pb isotope compositions of shale-hosted Zn-Pb deposits located within the Canadian Cordillera miogeocline. The new data is plotted together with previously determined Pb isotopic compositions for other sulphide occurrences in the area (discussed later). Analyses of feldspar from intrusions throughout the region generally yield quite consistent Pb isotopic compositions (Table 3.2), with the exception of one sample (SH-011E-INT) collected from the Mt. Billings, which returned significantly more radiogenic values. The Pb isotopic compositions (n=19) range from 19.397 to 19.651 for 2 0 6 Pb/ 2 0 4 Pb, 15.697 to 15.829 for 2 0 7 Pb/ 2 0 4 Pb, 39.461 to 39.883 for 2 0 8 Pb/ 2 0 4 Pb, 0.805 to 0.811 for 2 0 7 Pb/ 2 0 6 Pb, and 2.020 to 2.037 for 2 0 8 Pb/ 2 0 6 Pb. Analyses for sulphide samples collected from spatially associated mineral occurrences also yield similar Pb isotopic compositions (Table 3.2). Exceptions include three samples (SH-073-1, SH-070-1 and -2) from the Tai and Cali mineral occurrences at the north end of the Mt. Billings Batholith (Fig. 3.2), which returned the least radiogenic 2 0 7 Pb/ 2 0 6 Pb, and 2 0 8 Pb/ 2 0 6 Pb ratios from this study. The majority of analyses (n=9) yield narrow ranges of 2 0 6 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 4 Pb, 2 0 8 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 6 Pb, and 2 0 8 Pb/ 2 0 6 Pb ratios with values of 19.396 to 19.615, 15.711 to 15.904,39.391 to 40.039, 0.806 to 0.811, and 2.024 to 2.041, respectively. 69 Discussion The feldspar Pb isotopic compositions are highly radiogenic, plotting above the Shale Curve of Godwin and Sinclair (1982), indicating the intrusions are largely, i f not entirely, product of the partial melting of Selwyn Basin-like sedimentary rocks (see Chapter 2 for further discussion). Pb isotopic compositions of most sulphide occurrences proximal to the plutons are very similar to feldspar Pb isotopic compositions from the plutons themselves. This is consistent with the metals having been derived mainly from the plutons. In the case of the past producing Sa Dena Hes Zn-Pb-Ag skarn, sulphide Pb compositions plot within the cluster of Cretaceous intrusion feldspars and associated sulphides. Rare felsic and andesitic dykes are known to be associated with, and possibly related to mineralization at the mine but have not been dated directly. The similarity in Pb ratios suggests that the deposit is also mid-Cretaceous in age. Lead isotopic compositions from the' Quartz Lake (Macmillan) occurrence in the southeastern part of the study area (Fig. 3.2) are much more radiogenic than any of the other sulphides or feldspars from the area. The implications of this are uncertain. It could indicate that the metals in the Quartz Lake deposit were derived from an intrusive phase that is much more radiogenic than any recognized thus far in the study area, that the metals were derived mainly from sedimentary sources and are completely unrelated to the plutons, or possibly represent a mixture of igneous and sedimentary Pb. Lead isotopic compositions of sulphides from the Hyland Gold occurrence, a gold-rich, base-metal poor deposit located near the Quartz Lake occurrence, are not yet available. This occurrence has been suggested to represent a proximal style of intrusion-related mineralization. If this model is correct, the Hyland gold mineralization would be expected to yield Pb isotopic compositions intermediate between the Quartz Lake values and the cluster of Cretaceous intrusive feldspars and associated sulphides. 70 The sulphide Pb isotopic compositions are very different from typical SEDEX-type Pb-Zn occurrences in the Selwyn Basin (Fig. 3.3), indicating that none of the occurrences represent remobilized SEDEX-type mineralization. Conclusions The primary goal of this study was to compare and contrast the Pb isotope compositions of feldspars from various intrusions and sulphides from associated precious- and base metal deposits and occurrences within the southeastern TGP in order to investigate possible linkages between magmatism and mineralization. Results from this study indicate that the metals in many mineral deposits (and prospects) in the region are mostly derived from the mid-Cretaceous TGP intrusions. In an area with such voluminous magmatism, these results also serve to highlight the exploration potential throughout the study area. References Deklerk, R. (compiler) 2003. Yukon MINFILE - A database of mineral occurrences. Yukon Geology Survey, CD-ROM. Godwin, C.I. and Sinclair, A.J . 1982. Average lead isotope growth curves for shale-hosted zinc-lead deposits, Canadian Cordillera. Economic Geology 77: 208-211. Godwin C L , Gabites, J.E. and Andrew, A. 1988. Leadtable: A galena lead isotope database for the Canadian Cordillera. In: British Columbia Ministry of Energy and Mines and Petroleum Resources Paper 1988-4, 214. Gordey, S.P. and Anderson, R.G. 1993. Evolution of the northern Cordillera Miogeocline, Nahanni Map Area (1051), Yukon and Northwest Territories. Geological Survey of Canada Memoir 428, 214 p. Gordey, S.P. and Makepeace, A.J . 1999. Yukon Digital Geology, S.P. Gordey and A.J . Makepeace (comp.); Geological Survey of Canada, Open File D3826, and Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, Open File 1999-1(D). 71 Gordey, S.P. and Makepeace, A.J . (compilers) 2001. Bedrock Geology, Yukon Territory. In: Geological Survey of Canada, Open File 3754 and Exploration and Geological Services Division, Yukon Indian and Northern Affairs Canada, Open File 2001-1, scale 1:1 000 000. Hart, C.J.R., McCoy, D.T., Goldfarb, R.J., Smith, M . , Roberts, P., Hulstein, R., Bakke, A . A . and Bundtzen, T.K. 2002. Geology, exploration and discovery in the Tintina Gold Province, Alaska and Yukon. In: Society of Economic Geologists Special Publication 9, Integrated Methods for Discovery: Global Exploration in the 21 s t Century, p.241-274. Krogh, T.E. 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochimica et Cosmochimica Acta 46: 637-649. Lang, J.R., Thompson, J.F., Mortensen, J.K., Baker, C.J.R., Coulsen, I.M., Duncan, R. and Maloof, T. 2001. Tombstone-Tungsten Belt. In: Lang, J. (ed.), Regional and System-scale controls on the formation of copper and/or gold magmatic-hydrothermal mineralization. Mineral Deposit research Unit, Special Publication Number 2, January 2001, 115p. Mortensen, J.K., Hart, C.J.R., Murphy, D.C. and Heffernan, S. 2000. Temporal evolution Of Early and mid-Cretaceous magmatism in the Tintina Gold Belt. In: The Tintina Gold Belt: Concepts, Exploration and Discoveries, British Columbia and Yukon Chamber of Mines, Special Volume 2, p. 49-58. Thompson, J.F.H., Sillitoe, R.H., Baker, T., Lang, J.R. and Mortensen, J.K. 1999. Intrusion-related gold deposits associated with tungsten-tin provinces. Mineralium Deposita 34 No.4:323-334. Thirwall, M.F. 2000. Inter-laboratory and other errors in Pb isotope analyses investigated using a 207p b _204 p b d o u b l e s p i k e C h e m i c a l Geology 163: 299-322. 72 ZJ to 00 g ZJ T3 T 3 CD CZ E CD X CD to CD O tz CD i i Z J o o o TD CZ CD w io o o_ CD " D CD to to CZ o o to CD 7 3 O to CD T3 tz CD tz g CD O O co re to _CD E 1 CD CO CD cn .E 1 -*—> to CD LU - c z 1 O .9 ~ x: CD t 8 o to CD ' T3 O E E o O CD O JS CD CD O tz CD •<-• iz io 3 °, O Q. O CD O a CN • CO o I X CO CO o CN -cr-CO o o • X CO o" I---o a> CN o i X CO > > L O L O o o LU X X X CO CO CO X CO o CD C L to o o CD C L to O C L C L O CD C L to O O CL CL V. 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O O CO Q_ O E a? CD CO J = D ) 8 J. CD O ) <2 CO" . O ^ O CD 4= > s Q . O a. S° co a. CO .cu " D CO CD * " ^—i • • O CO 3 C " O O C * = O CO O > CO CD co c to ™ CD < ~ 74 Figure 3.1. Map of Alaska State and Yukon Territory showing the extent of the Tintina Gold Province and the locations of significant gold deposits and occurrences within (after Mortensen et a l , 2000). The main study area is outlined at right. 75 Figure 3.2. Map of study area showing the location of intrusion-related deposits discussed in text and occurrences and the distribution of early and mid-Cretaceous intrusions (light shaded areas). Stars denote the location of feldspar (mineral separates) samples used in this study. Dashed lines separate designated plutonic suites as discussed in text and the dark shaded area outlines the extent of the overlapping Tungsten suite. Dotted lines represent major roads. Map modified from Gordey and Makepeace, 1999. 76 2 0 7 p b / 2 0 6 p b 0.85 |_ A feldspar separate (this study) • sulphide analysis (this study) • Sa Dena Hes (Mt. Hundere) - #105A 012 • Nar-#1051 004 • Fir Tree (Norquest) - 105H 029 0.84 — i — 0.83 — i — 0.82 0.81 i 0.80 i 0.79 0.78 0.77 A Pb data from the Quartz Lake (McMillan) deposit Pb data from Paleozoic (Zn-Pb) SEDEX-type occurences Representative uncertainty ellipses for data from this study: sulphide analyses ff feldspar analyses I I 1.97 1.98 H 1.99 2.00 2.01 2.02 2 .03 2.04 2.05 2.06 2 .07 2.08 2 .09 ro o CO T I D" M O T I Figure 3.3. 2 0 7Pb/ 2 0 6Pb versus 2 0 8Pb/ 2 0 6Pb plot of Pb data from this study plotted and similar data compiled from Godwin et al. (1988) for other mineral occurrences and deposits from within the study area. Pb data for the Paleozoic SEDEX-type occurrences includes data from the Howards Pass, Matt Berry, Mel-Hoser, Maxi, and Pas occurrences. Shale Curve of Godwin and Sinclair (1982) is shown for reference. 77 Chapter 4 Summary and Directions for Future Research Summary Mid-Cretaceous magmatism in the Tintina Gold Province (TGP) of southeastern Yukon and southwestern Northwest Territories represents some of the most prolific, yet poorly studied granitic magmatism of the northern Canadian Cordillera. Geochronological, lithogeochemical, and isotopic data produced during the course of this study provides a temporal and geochemical framework for mid-Cretaceous magmatism in the southeastern TGP. The data confirm the continuation of known plutonic suites from the northern and western portions of the TGP, and provide new insights into the overall tectonomagmatic evolution of the region. These results will also bear directly on genetic and exploration models developed for associated precious- and base-metal mineralization. U-Pb geochronology identified the same 'inboard younging' pattern of magmatism that was previously recognized in the northern and western portions of the TGP (Mortensen et al., 2000). Within the study area, Anvil plutonic suite (APS; >100 Ma) magmatism is for the most part limited to the westernmost Mt. Billings Batholith. The majority of intrusions, including the large Coal River and Hole-in-the-Wall batholiths, are considered part of the Tay River plutonic suite (TRPS; -99 to -95 Ma). The strongly peraluminous two-mica Tungsten plutonic suite (WPS; -99 to -95 Ma) does not appear to extend south of the Cantung area. The Tungsten plutonic suite seems restricted to a region that shows the most evolved isotopic signatures, possibly suggesting a direct correlation with the nature of the underlying basement rocks. The 78 economically significant Tombstone plutonic suite (TPS; <~95 Ma) continues to the extreme southeast end of the TGP, including the Big Charlie and McLeod plutons. The dominantly granitic and granodioritic intrusions are high-K calc-alkaline in nature with mixed I- and S-type characteristics. The intrusions tend to become less peraluminous with decreasing age. Primitive mantle normalized REE patterns are virtually identical for the APS, TRPS and TPS, displaying steep profiles with negative Nb, Eu, and Ti anomalies. These characteristics are generally ascribed to I-type volcanic arc or subduction-related granitoids, however this signature is probably partially, i f not entirely, inherited from the partial melting or assimilation of crustal material previously formed in a subduction setting. TPS intrusions show more pronounced LREE-enrichment and less prominent Eu anomalies. There is an increase in overall REE abundance with decreasing age (APS -> TRPS-> TPS) which also correlates with a general increase from -700° to ~800°C in calculated zircon saturation temperatures The high initial Sr/ Sr, low eNd, radiogenic Pb compositions, Mesoproterozoic to Archean T D M ages, and the peraluminous nature of the granitoids indicate a dominantly crustal source for these magmas. Without contemporaneous mafic volcanism, it is difficult to conclusively constrain the paleotectonic setting in which the mid-Cretaceous magmatism occurred. The data presented here provide some insight into the nature of the tectonic setting; however for such large region, it can provide little more than generalities. Lead isotope compositions of feldspars separated from various mid-Cretaceous intrusions and of sulphides from a number of precious- and base metal deposits are very similar and indicate that the metals in many mineral deposits (and prospects) throughout the region are mostly derived from the mid-Cretaceous TGP intrusions. 79 Directions for Future Research Although this thesis has contributed to the understanding of the magmatic, tectonic, and metallogenic understanding of the TGP in the eastern Selwyn Basin, numerous questions remain unanswered and provide directions for future research. Perhaps the most obvious question to emerge from this study is: What happens to the plutonic suites south of the study area? The three most obvious answers are: 1) the TGP, including the plutonic suites recognized in the study area, terminates or is truncated; 2) the TGP continues to the south in a similar fashion and intrusions are not exposed at current levels of erosion; and 3) plutonic suites within the TGP converge, swing to the southwest, and wrap around the basement promontory where the narrow margin has focused the locus magmatism. Indeed this is not a trivial question to answer. Studies similar to this one on intrusions that may be the southward continuation of the TGP (currently those west of the Tintina Fault in south-central Yukon) will be required to resolve this. The shear volume of magmatism suggests that this portion of the crust remained hot for a significant period of time. The combination of expanded U-Pb dating, particularly for the larger batholiths, and regional Ar-Ar dating would be able to provide insight into whether the magmatism was essentially a continuum or was sharply episodic, as appears to have been the case further to the northwest (Mortensen et al., 2000). One of the most challenging directions for future research will be to address the change in geochemistry and isotopic compositions with age, and geographic location. In particular, the question of whether the shift towards higher REE content and generally 'less-evolved' isotopic signatures reflects differences in the nature of underlying crust, a tectonic process whereby intrusions incorporate larger amounts of primitive material or i f it represents a shift in the melt dissolution-kinetics (i.e., more wholesale melting of crustal material). This problem will require 80 integrating new geochemical and geochronological studies on the regional scale with detailed mineralogical studies. Additional geothermobarometry, geochemical, and isotopic studies, including Hf and 18 8 O, on both intrusive rocks and potential source rocks will provide the best means to further constrain the question of magma sources. The combination of these studies with detailed mapping to examine issues such as the nature of emplacement will ultimately lead to the development of a robust tectonomagmatic model for the region Tracing the economically important Tombstone plutonic suite (TPS) is a challenge left for the exploration industry. Within the Intrusion-related Gold System (IRGS) model, the depth of emplacement exhibits a strong control on not only the target type but the associated pathfinder elements as well. Furthermore, factors such as the level of erosion further complicate exploration for IRGS. Integrated geochemical and geophysical techniques will be required to test the eastward and southward limits of the TPS. References Mortensen, J.K., Hart, C.J.R., Murphy, D.C. and Heffernan, S. 2000. Temporal evolution of Early and mid-Cretaceous magmatism in the Tintina Gold Belt. In: The Tintina Gold Belt: Concepts, Exploration and Discoveries, British Columbia and Yukon Chamber of Mines, Special Volume 2, p. 49-58. 81 Appendix 1 Analytical Precision The precision of lithogeochemical data obtained during the course of this study was monitored by replicate analyses of Mineral Deposit Research Unit in-house standards P - l (granodiorite from the Coast Plutonic Complex) and BAS-1 (basalt from near Cheakamus, British Columbia). Analytical data obtained from these samples was compared to a mean of 5 previous repeat analyses and are presented in Table A . 1. Duplicate analyses of these samples are precise and are within two standard deviations of the standard values. 82 Table A.l. Mean values and duplicate analyses of standards P-l and BAS-1. P-l (n=5)* mean std deviation P-i-A P-l-B BAS-1 (n=5) mean std deviation BAS-1-A BAS-1 -Major elements (wt. %) Si0 2 70.96 0.10 69.90 69.83 53.56 0.36 52.41 52.45 T i 0 2 0.38 0.00 0.38 0.80 1.31 0.01 1.33 1.31 A1 2 0 3 14.10 0.06 14.27 14.24 15.12 0.07 15.06 15.08 Fe 2 0 3 3.90 0.00 3.89 3.85 11.16 0.05 11.09 11.10 Fe 2 0 3 2.34 0.08 2.25 2.25 8.86 0.12 8.73 8.76 MnO 0.08 0.00 0.08 0.08 0.14 0.00 0.15 0.14 MgO 1.11 0.01 0.91 0.93 7.35 0.05 7.10 7.11 CaO 3.49 0.02 3.35 3.38 8.28 0.05 8.28 8.30 Na 2 0 3.80 0.00 3.78 3.80 3.28 0.04 3.05 3.06 K 2 0 2.12 0.01 2.19 2.21 0.56 0.02 0.61 0.60 P2O5 0.08 0.04 0.07 0.08 0.22 0.00 0.23 0.23 TOTAL 100.56 0.22 99.28 99.20 100.96 99.15 99.26 Trace and rare earth elements (ppm) Ag 0.3 <1 <1 0.28 0.08 <1 <1 Ba 724 8 825 932 194 18.55 209 184 Ce 28 1.26 25 29.5 21.8 0.75 20.5 20.5 Co 6.2 0.4 5.5 6.0 42.2 0.4 39.0 41.0 Cs 1.2 0.1 1.1 1.1 0.1 0.0 <0.1 <0.1 Cu 15.5 5.5 5.0 5.0 59.0 0.8 55.0 65.0 Dy 3.3 0.2 2.3 3.1 3.3 0.2 2.6 2.7 Er 2.1 0.1 1.9 2.1 1.5 0.1 1.4 1.4 Eu 0.8 0.0 0.7 0.7 1.3 0.1 1.0 1.0 Ga 15.0 1.1 12.0 13.0 19.6 1.0 16.0 17.0 Gd 3.1 0.1 2.7 2.5 3.8 0.1 3.0 3.3 Hf 3.8 0.1 3.0 4.0 2.4 0.1 2.0 1.0 Ho 0.7 0.0 0.5 0.5 0.6 0.0 0.5 0.5 La 13.2 0.4 14.0 16.0 9.3 0.4 10.5 9.5 Lu 0.4 0.0 0.3 0.3 0.2 0.0 0.1 0.1 Nb 3.8 0.2 2.0 3.0 8.2 0.5 6.0 6.0 Nd 13.0 0.6 10.5 12.0 13.6 0.8 11.0 11.5 Ni - - <5 <5 172.0 1.9 160.0 170.0 Pb 10.2 1.5 15.0 5.0 4.4 4.3 40.0 <5 Pr 3.4 0.1 2.6 2.6 3.0 0.1 2.2 2.3 Rb 50.4 3.1 40.4 43.2 7.0 0.1 5.2 5.6 Sm 2.9 0.1 1.9 2.7 3.5 0.5 2.5 2.4 Sn 2.4 0.9 1.0 1.0 1.2 0.2 1.0 1.0 Sr 256.0 4.9 194.5 207.0 502.0 7.5 401.0 426.0 Ag 0.3 0.0 0.4 0.5 0.5 0.0 0.5 0.5 Tb 0.5 0.0 0.4 0.5 0.6 0.0 0.5 0.5 Th 4.4 - <0.5 <0.5 0.8 0.0 <0.5 <0.5 RT1 0.3 - <0.5 <0.5 0.1 0.0 <0.5 <0.5 Tm 0.4 0.0 0.3 0.3 0.2 0.0 0.1 0.1 U 1.5 0.1 1.5 1.5 0.3 0.0 <0.5 <0.5 V 58.2 0.4 45.0 45.0 152.0 1.1 125.0 140.0 Y 22.8 0.5 18.0 19.0 18.4 1.0 14.0 14.5 Yb 2.5 0.2 1.9 2.0 1.4 0.1 1.2 1.1 Zn 44.0 0.9 30.0 40.0 91.4 1.0 85.0 90.0 Zr 126.0 10.2 126.0 123.0 94.5 2.2 93.0 90.0 P-l and BAS-1 are MDRU standards. * Mean values of P-l and BAS-1 are based on the average of 5 previous repeat analyses. A and B are duplicate analyses of the standards that were used to test for precision. 

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