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Mineralogical and geochemical study of the True Blue aquamarine showing, Shark Property, southern Yukon… Turner, David James 2006

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M I N E R A L O G I C A L A N D G E O C H E M I C A L S T U D Y O F T H E T R U E B L U E A Q U A M A R I N E SHOWING, S H A R K PROPERTY, SOUTHERN Y U K O N TERRITORY by . D A V I D J A M E S T U R N E R B . Sc., University of Victoria, 2003 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E OF M A S T E R OF S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S (Geological Sciences) T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A December 2005 © David James Turner, 2005 ABSTRACT Aquamarine of distinctly dark blue colour was discovered during the summer of 2003 in the Pelly Mountains, southern Yukon Territory, Canada. The beryl is found within quartz veins that fill sigmoidal tension gashes, which cut a Mississippian syenite. The True Blue showing is differentiated from other beryl occurrences in the northern Cordillera by the colour of the beryl, host rock, mineral associations, timing, and mineralizing fluid. The syenite was emplaced within an extensional setting into undeformed Paleozoic platform sediments of the Cassiar Platform and felsic volcanics of the Pelly Mountain Volcanic Belt. Post late-Triassic tectonics resulted in a number of northeast-directed thrust panels that were subsequently cut by Cretaceous granitic magmatism. Accessory minerals in the veins include siderite, ankerite, allanite, fluorite, and minor albite, various sulfide minerals, and Fe-Ti-Nb oxide minerals. Electron microprobe analyses of beryl (n = 192) revealed that FeO values range up to 5.92 wt.%, N a 2 0 up to 2.66 wt.%, MgO up to 3.42 wt.%, and CaO up to 0.11 wt.%, while little to no Cr, Sc or V were detected. The darkest blue beryl has the highest concentrations of FeO. Allanite is of the Ce-dominant variety, contains up to 26 wt.% REE2O3, and exhibits Fe 2 + > Fe3 +. Fluorite from several veins that co-precipitated with beryl has been dated using Sm-Nd geochronology at 171.4 ±4.8 Ma. In situ and whole-mineral 5 1 8 OSMOW values from beryl and whole-mineral 18 ' S OSMOW values from quartz are variable and temperature estimates derived from this data suggests fluid temperatures between -275 and -425 °C. Conventional gem beryl formation models, and consequently exploration parameters, applied in Yukon invoke late-stage magmatic fluids. Evidence gathered in this study points to a metamorphic origin for the mineralizing fluid and local derivation of vein constituents, thus differentiating the fluids at True Blue from other intrusion-related beryl-forming fluids in the northern Cordillera. Keywords: aquamarine, beryl, geology, mineralogy, geochemistry, geochronology, fluid inclusions, stable isotopes, Pelly Mountains, Yukon Territory, Canada. ii T A B L E O F C O N T E N T S A B S T R A C T i i T A B L E O F C O N T E N T S i i i L IST O F T A B L E S v i LIST O F F I G U R E S v i i A C K N O W L E D G E M E N T S x C O N T R I B U T I O N S xi C H A P T E R 1. I N T R O D U C T I O N 1 1.1. Overview 1 1.2. Project significance and scope 2 1.3. Geochemistry of beryl/aquamarine 4 1.4. Aquamarine deposit models 5 C H A P T E R 2. R E G I O N A L A N D L O C A L G E O L O G Y 9 2.1. Introduction 9 2.2. Regional geology of the Pelly Mountains 12 2.3. Regional metallogeny of the Pelly Mountains 14 2.3.1. Overview 14 2.3.2. Mississippian 14 2.3.3. Cretaceous 15 2.4. Loca l setting at the True Blue showing 15 2.4.1. The Shark property .,15 2.4.2. Work history 16 2.4.3. Fieldwork 17 2.4.4. Geological unit descriptions 18 Sedimentary rocks 18 Igneous rocks 21 C H A P T E R 3. G E O C H E M I S T R Y OF T H E S Y E N I T E H O S T R O C K S 22 3.1. Whole-rock geochemistry 22 3.1.1. Major elements 22 3.1.2. Trace elements : 28 3.1.3. Interpretation of whole-rock geochemistry 28 3.1.4. Petrogenesis 33 C H A P T E R 4. M I N E R A L O G Y O F T H E S Y E N I T E H O S T R O C K S 35 4.1. Introduction 35 4.2. Pr imary minerals 35 4.2.1. A l k a l i feldspars 35 4.2.2. Zircon 38 4.2.3. Possible destroyed primary minerals 42 4.3. Secondary minerals 42 4.3.1. A lka l i feldspars 42 4.3.2. Phyllosilicates 44 4.3.3. Carbonate minerals 49 4.3.4. Phosphate minerals 52 4.3.5. Oxide minerals 52 4.3.6. Quartz 52 i i i 4.3.7. Other accessory minerals 52 4.4. Interpretation of syenite mineralogy 55 4.4.1. Introduction 55 4.4.2. V M S assemblage 55 4.4.3. Flame perthite 56 4.4.4. Br ie f timeline of reactions / interpretation 57 C H A P T E R 5. M I N E R A L O G Y A N D G E O C H E M I S T R Y OF B E R Y L - B E A R I N G Q U A R T Z V E I N S 58 5.1. Overview of morphologies, textures and vein origins 58 5.2. Macroscopic morphology of veins at the True Blue showing 59 5.3. Mineralogy in order of abundance 59 5.3.1. Quartz ...... 59 5.3.2. Carbonate minerals 61 5.3.3. Beryl 62 Colour in beryl 68 5.3.4. Allanite 79 5.3.5. Fluorite 84 5.3.6. Albite 84 5.3.7. Oxide and sulfide minerals 84 5.4. Cathodoluminescence 84 5.5. F lu id inclusions 87 5.6. Interpretation and observations and origin of vein mass 87 5.6.1. Interpretation o f mineralogy and vein textures 87 5.6.2. Emplacement conditions 89 5.6.3. Origin of vein mass 89 5.6.4. Gradients inducing mass transfer 89 5.6.5. Element mobility and sources 90 Common elements 90 Rare elements (REE + Be + Ti) 91 5.6.6. Mechanisms o f precipitation 94 5.7. Summary : 94 C H A P T E R 6. S T A B L E ISOTOPES 96 6.1. Purpose 96 6.2. Methodology 96 6.3. Data 96 6.3.1. In situ '....96 6.3.2. Whole-mineral '. ' 99 Oxygen 99 Carbon 99 6.4. Interpretation of oxygen isotopes 99 6.5. Interpretation of carbon isotopes : 103 6.6. Summary 103 C H A P T E R 7. G E O C H R O N O L O G Y 104 7.1. Introduction 104 7.2. Accessory mineral dating 104 7.3. Previous studies 105 iv 7.4. Geochronology of the Pelly Mountains 106 7.5. Sm-Nd geochronology of fluorite 106 7.5.1. Methodology 106 7.5.2. Data 109 7.5.3. Interpretation 109 7.6. U-Pb geochronology of allanite 110 7.7. Summary 111 C H A P T E R 8. D E P O S I T M O D E L 112 8.1. Introduction 112 8.2. Deposit model 112 C H A P T E R 9. E X P L O R A T I O N P A R A M E T E R S A N D T A R G E T S 116 9.1. Introduction 116 9.2. Parameters 116 9.2.1. Distal '. : 118 9.2.2. Medial 118 9.2.3. Proximal 119 9.3. Targets within Pelly Mountains 119 9.3.1. Noklui t and Kay 120 9.3.2. Mount Vermil l ion - W o l f 120 9.4. Targets within Yukon Territory 121 9.4.1. M c E v o y Platform 121 9.4.2. Selwyn Basin 121 9.5. Targets outside Y u k o n Territory 122 9.5.1. Tauern Window, Southeast Alps 122 9.6. Summary 123 C H A P T E R 10. C O N C L U S I O N 125 10.1. Conclusions 125 10.2. Future research 125 B I B L I O G R A P H Y 127 A P P E N D I X 1: Analytical procedures 143 A P P E N D I X 2: Electron microprobe data • C D A P P E N D I X 3: Whole-rock geochemical data C D A P P E N D I X 4: Energy dispersive spectrometry spectra C D A P P E N D I X 5: U-Pb geochronological data - C D A P P E N D I X 6: Extractor sample locations and descriptions C D v L I S T O F T A B L E S T A B L E 1. S E L E C T E D W H O L E - R O C K G E O C H E M I S T R Y O F S Y E N I T E F R O M T H E T R U E B L U E S H O W I N G 2 3 T A B L E 2 . W H O L E - R O C K G E O C H E M I S T R Y O F P R E V I O U S L Y A N A L Y Z E D S Y E N I T E N E A R T H E T R U E B L U E S H O W I N G 2 5 T A B L E 3 . C I P W N O R M A L I Z A T I O N F O R S Y E N I T E S A M P L E S 2 6 T A B L E 4 . C I P W N O R M A L I Z A T I O N F O R H I S T O R I C A L S Y E N I T E S A M P L E S 2 7 T A B L E 5. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F F E L D S P A R F R O M T H E T R U E B L U E S H O W I N G 3 7 T A B L E 6. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F Z I R C O N F R O M T H E T R U E B L U E S H O W I N G 4 0 T A B L E 7. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F P H Y L L O S I L I C A T E S F R O M T H E T R U E B L U E S H O W I N G 4 6 T A B L E 8. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F C A R B O N A T E M I N E R A L S F R O M T H E T R U E B L U E S H O W I N G 51 T A B L E 9. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F O X I D E M I N E R A L S F R O M T H E T R U E B L U E S H O W I N G •. 5 4 T A B L E 10 . S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F B E R Y L F R O M T H E T R U E B L U E S H O W I N G 6 4 T A B L E 11 . S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F A L L A N I T E F R O M T H E T R U E B L U E S H O W I N G 81 T A B L E 12. I N - S I T U 5 I 8 0 S M o w D A T A O F B E R Y L F R O M T H E T R U E B L U E S H O W I N G 9 8 T A B L E 1 3 . W H O L E M I N E R A L 5 , 3 C P D B A N D 8 , 8 0 S M O W V A L U E S F R O M S E L E C T E D P H A S E S W I T H I N M I N E R A L I Z E D V E I N S 1 0 0 T A B L E 14. S E L E C T E D G E O C H R O N O L O G I C A L D A T A F R O M Y U K O N A G E D A T A B A S E 1 0 7 T A B L E 15 . S M - N D G E O C H R O N O L O G I C A L D A T A F O R F L U O R I T E F R O M T H E T R U E B L U E S H O W I N G 1 1 0 T A B L E 16 . S U M M A R Y O F E X P L O R A T I O N P A R A M E T E R S A S D E S C R I B E D I N T E X T 1 1 7 vi LIST OF F I G U R E S F I G U R E 1. General location of the True Blue showing within Canada 1 F I G U R E 2. Shark property claim block, southern Yukon Territory 3 F I G U R E 3. Crystal structure of beryl 4 F I G U R E 4. Conceptual schematic of pegmatite 7 F I G U R E 5. Conceptual schematic o f evolved granite, greisen, and vein stockwork ; 7 F I G U R E 6. Terrane maps of Yukon Territory and Western Canada 10 F I G U R E 7. Stratigraphic cross section of the Pelly Mountains in the Quiet Lake map area (NTS 105F) 11 F I G U R E 8. Conceptual cartoon of Devono-Mississippian tectonics o f western Canadian Cordillera 11 F I G U R E 9. Simplified geological map of the Pelly Mountains, Yukon Territory 13 F I G U R E 10. Geological map of the True Blue showing in the Pelly Mountains, Yukon Territory 19 F I G U R E 11. Geological photographs from the Shark property 20 F I G U R E 12. Shand Index diagram from syenite samples 30 F I G U R E 13. Albitization effects on whole rock geochemistry of syenite 30 F I G U R E 14. Tectonic discrimination diagrams of syenite 31 F I G U R E 15. Rare earth and trace element diagrams for syenite 32 F I G U R E 16. Photomicrographs of albitized K-feldspar in syenite 36 F I G U R E 17. Photomicrographs o f altered zircon in syenite 39 F I G U R E 18. Diagrams of zircon crystal chemistry 41 F I G U R E 19. Photomicrographs of flame perthite in syenite 43 F I G U R E 20. Photomicrographs o f secondary biotite in syenite 45 F I G U R E 21. Photomicrographs o f secondary chlorite in syenite 45 F I G U R E 22. Diagram of biotite crystal chemistry 47 F I G U R E 23. Diagram of biotite crystal chemistry 47 F I G U R E 24. Diagram of biotite crystal chemistry 47 F I G U R E 25. Diagram o f biotite crystal chemistry 48 F I G U R E 26. Diagram of biotite crystal chemistry 48 vii F I G U R E 27. Photomicrographs of secondary carbonate in syenite 50 F I G U R E 28. Photomicrographs of secondary phosphate in syenite 53 F I G U R E 29. Photomicrographs of secondary pyrite in syenite 53 F I G U R E 30. Photomicrographs of textures in quartz veins cutting syenite 60 F I G U R E 31. Photomicrograph of quartz vein with euhedral siderite cutting syenite 61 F I G U R E 32. Photomicrographs and back-scattered electron images of complex composi-tional zoning and. multiple anhedral and euhedrdal cores in beryl 63 F I G U R E 33. Diagrams of crystal chemistry of beryl from quartz veins 65 F I G U R E 34. Mossbauer spectra collected at 14 K from dark blue beryl 67 F I G U R E 35. Photographs o f beryl and associated minerals in quartz veins 69 F I G U R E 36. Diagrams of crystal chemistry of beryl from quartz veins 71 F I G U R E 37. Diagrams o f crystal chemistry of beryl from quartz veins 71 F I G U R E 38. Diagrams of crystal chemistry of beryl from quartz veins 73 F I G U R E 39. Diagrams o f crystal chemistry o f beryl from quartz veins 73 F I G U R E 40. Diagrams of crystal chemistry of beryl from quartz veins 74 F I G U R E 41. Diagrams of crystal chemistry o f beryl from quartz veins 76 F I G U R E 42. Diagrams of crystal chemistry of beryl from quartz veins 77 F I G U R E 43. Diagrams of crystal chemistry of beryl from quartz veins 78 F I G U R E 44. Photographs and B S E images of allanite from quartz veins 82 F I G U R E 45. Diagram o f crystal chemistry of allanite from quartz veins 83 F I G U R E 46. Diagrams of crystal chemistry of allanite from quartz veins 83 F I G U R E 47. Photomicrographs o f fluorite from quartz veins 85 F I G U R E 48. Rare earth element diagram (with back-scattered electron image inset) for fluorite from quartz veins 85 F I G U R E 49. Photomicrographs o f albite from quartz veins 86 F I G U R E 50. Back-scattered electron and cathodoluminescence images of fluorite liberated from quartz veins 86 F I G U R E 51. Simplified paragenetic sequence for mineral assemblages developed prior to and during vein emplacement 88 viii F I G U R E 52. F I G U R E 53. F I G U R E 54. F I G U R E 55. F I G U R E 56. Back-scattered electron images for beryl crystals analyzed by in situ methods for stable oxygen isotopes 97 Pairing o f stable oxygen isotope data for beryl from quartz veins with that from quartz from quartz veins and derived temperature estimates 102 Geochronological data from the YukonAge database (Breitsprecher et al. 2003) plotted on a simplified geological map of the Pelly Mountains, Yukon Territory '. 108 Samarium-Neodymium isochron from fluorite collected from quartz veins cutting syenite at the True Blue showing 110 Schematic o f proposed deposit model for mineralization at the True Blue showing, southern Yukon Territory, Canada 115 ix Acknowledgements I would like to thank Dr. Lee Groat, Dr. Jim Mortensen and Dr. Mati Raudsepp for guidance and supervision during the course of this study. I would also like to thank many of the members o f P C I G R for discussions regarding various hypotheses, as well as all of the persons that provided fieldwork assistance. The staff and students o f the Department o f Earth and Ocean Sciences are greatly thanked for their contributions alonj the way. This project was made financially possible by funds from the National Sciences and Engineering Research Council , True North Gems, Inc., and the Yukon Geological Survey. Most of all I would like to thank my wonderful wife who provided endless support and love throughout this project. x Contributions David J. Turner (University of British Columbia): mapping, petrography, electron microprobe analysis, scanning electron microscope data (e.g., E D S , imaging), most sample preparation, literature research and interpretation, overall data interpretation. Dr. Lee A . Groat (University of British Columbia): Thesis supervision. Dr. James K . Mortensen (University of British Columbia): Committee member, U-Pb geochronology data. Dr. Mat i Raudsepp (University of British Columbia): Committee member, microprobe assistance. Dr. Dominique Weis (University of British Columbia): Sm-Nd geochronology data. Dr. Richard Friedman (University of British Columbia): U-Pb geochronology data. Dr. Gaston Giuliani (Centre de Recherches Petrographiques et Geochimiques): In situ oxygen isotope data. Dr. Robert L . Linnen (University of Waterloo): Fluid inclusion data and interpretation. Dr. M . Darby Dyar (Mount Holyoke College): Mossbauer data. True North Gems Inc.: Financial and field support. N S E R C : Financial support. Archer, Cathro & Associates (1981) Limited: Field support. A L S Chemex: Whole rock geochemical data. Queen's Facility for Isotope Research: Whole-mineral stable isotope data. Vancouver Petrographies Ltd.: Polished section preparation. x i CHAPTER 1 INTRODUCTION Overview Dark blue gem-quality beryl (var. aquamarine) was discovered during the summer of 2003 at the True Blue mineral showing on True North Gems Incorporated's Shark claims in the Pelly Mountains, south-central Yukon Territory, Canada (61°30'N, 132°30'W, NTS 105F/8, 9, and 10; Figure 1). The discovery occurred as a result of a regional gemstone exploration program conducted by W. Wengzynowski and L .A . Groat and funded by True North Gems Incorporated. The Shark claims were staked prior to ground exploration on the basis that light-blue beryl was discovered by W.D. Eaton of Archer, Cathro & Associates (1981) Limited during a uranium exploration program in 1976. Over 100 occurrences of beryl were documented during the 2003 field season and more than 100 additional occurrences were discovered during the 2004 field season. Beryl crystals occur in a swarm of closely spaced quartz ± siderite ± fluorite ± allanite veins which fill tension gashes ranging in thickness from 0.5 to 20 cm, and locally comprise up to 30% of the rock on the m scale. The veining is most concentrated near the upper contact between an altered syenite intrusive body and Paleozoic metavolcanic rocks, pelite, and carbonate country rocks. Y U K O N T E R R I T O R Y Da#soo City HaJn#s Junctor^  ; Figure 1. General location of the True Blue showing within Yukon Territory and Canada. 1 Three main zones host gem-quality beryl in bedrock and locally derived talus. These zones occur within a -600 m radius area over a -200 m range in elevation. O f these three zones, "Shark B o w l " is the largest and contains the highest concentration of beryl (Figure 2). The colour of the beryl ranges from green to yellow and pale to dark blue. The 2003 field season yielded 24 specimen samples and two bulk samples weighing -115 and 60 kg, the smaller of which (GS2) was processed by conventional crushing. A total of 57.9 g of dark blue material was recovered from GS2 and five stones weighing up to 0.82 carats were faceted (Rohtert et al. 2003). Efforts during the 2004 field season focused on extraction of less weathered beryl-bearing quartz veins and yielded 785 kg of concentrated vein material. Project significance and scope The main objectives of this study were to describe the geology and mineralogy of the True Blue beryl showing and to determine the origin of mineralization. Characterization of the system was achieved through geological mapping, petrography, whole-rock geochemistry, electron probe microanalysis, geochronology, oxygen isotope geochemistry, and fluid inclusion studies. The timing, nature, and geochemical affinity of the mineralizing fluid distinguish the True Blue gem-beryl showing from others in the Northern Cordillera. Its colour and mineralogical associations further distinguish this occurrence not only from those in the Northern Cordillera, but also globally. This study has also given insight into regional tectonics in southern Yukon and provided information pertaining to exploration for gem beryl of syntectonic origin and other syntectonic mineralization in Yukon Territory. A deposit model was constructed and exploration parameters applicable primarily to Yukon were then developed. •2 Figure 2. The Shark claims covering the True Blue showing, Pelly Mountains, southern Yukon Territory. Geochemistry of beryl/aquamarine Beryl is an aluminous beryllium cyclosilicate that commonly forms hexagonal prisms with basal pinacoids. It has a hardness of 7.5 to 8 on the Mohs scale, and is colourless when pure. A basal cleavage is present and fractures are described as conchoidal to splintery. Density of beryl ranges from -2.6 to -2.9 g/cm 3 , with variations being due to element substitutions. Fluorescence can be observed in beryl, and is most commonly ascribed to the presence of C r 3 + . The ideal chemical formula for beryl is in Be3Al 2Si60i8, but is better described by the general formula (Na,Cs)2x-Y+z c ( H 2 0 , H e , A r ) < 2 . ( 2 X - Y + z ) + N a r2(Be3.x L i Y D X - Y ) 0 ( A l , F e , S c , C r ) 3 + 2 . z ° ( F e , M g , M n ) 2 + z n [ S i 6 0 i 8 ] , where (Y<2), (X>Y), (Z<2) and (2X-Y+Z<2) (Cerny 2002 and references therein). Sites noted in the formula refer to the tetrahedrally coordinated sites of Si and Be (77 and T2, respectively), the octahedrally coordinated site of A l (O), and the distinct channel site (C) located along the c-axis within rings of six-membered SiCU tetrahedra (Figure 3). Two examples of complete replacement of A l at the O site have been noted for S c 3 + and F e 3 + , thereby defining distinct minerals with the beryl group. The Sc-dominant variety is light to dark blue bazzite (Grammaccioli et al. 2000) which has been : * I f W *- /mm la: <**• S i O , t e t r a h e d r a JMS , fla1 *> Channel sites 1* Jit Figure 3. Crystal structure of beryl viewed parallel to the c-axis (a) and perpendicular (b) to the c-axis (Modified from Hulburtand Klein 1977). 4 discovered in association with Niobium-Yttrium-Fluorine pegmatites ( N Y F Type; Cerny 1990). The Fe -dominant variety stoppaniite is also light blue in colour and has been reported to occur in sanidinites (Delia Ventura et al. 2000, and Ferraris et al. 1998). Almost all beryl crystals contain at least minor substitutions, typically of the aforementioned elements in the general formula, which generate the variety of colour displayed by this single mineral. The most familiar example is that of ° C r 3 + , which imparts a vibrant green to the variety of beryl known as emerald. It is generally agreed upon that the light blue colour of aquamarine is a result o f Fe, however the details of locations and valence states of Fe are more complicated and are discussed later. In the gemological realm, aquamarine is considered to be a type 1 gem, as defined by the Gemological Institute of America (GIA). The G I A defines type 1 gems as being essentially free of inclusions and therefore 'eye-clean'. Aquamarines of the finest quality can reach up to -1500 $ C D N per carat (1 gram = 5 carats). Aquamar ine deposit models Overview Much information regarding the distribution of elemental Be and Be mineralization is contained in reviews by Grew (2002a), Franz and Morteani (2002), Barton and Young (2002), and London and Evensen (2002). From these reviews and references therein common sources of Be in beryl deposits include pegmatites (e.g., Minas Gerais, Brazil) and evolved felsic igneous rocks with rare-element enrichments (e.g., Thor Lake, Canada). Other significant sources of Be can include organic material (e.g., M u z o and Chivor, Colombia), metapelitic rocks, and Mn-r ich rocks (e.g., Calatrava, Spain). Beryl of the aquamarine variety is typically found in pegmatites. Examples of these occurrences are found world over, but the finest gem quality aquamarine is sourced from Brazi l with other important locations in Russia, U S A , Madagascar, Sri Lanka, and Pakistan (Sinkankas 1981). Lesser amounts of aquamarine are found in greisens and epigenetic veins of Sn-W-Mo enriched granitoids, with many examples located in the Western Cordillera of North America. Aquamarines less commonly have a metamorphic origin, such as those found in the Alpine clefts and syntectonic veins of the Eastern Alps . 5 Pegmatitic Beryl occurrences associated, with pegmatites has been described in detail by Cerny (2002, 1991a, 1991b) and a number of other authors have also given summarized descriptions (e.g., Sinkankas 1981, Shigley and Kampf 1984, Walton 2004). In brief, aquamarine is typically found within rare-element-enriched pegmatites, more commonly of the Lithium-Cesium-Tantalum ( L C T Type; Cerny 1990) than N Y F Type. Although beryl has been noted to form from wall and border zones to cores, the highest quality crystals (i.e. large size, good transparency and colour) typically reside in open space cavities (Figure 4). Other minerals commonly found in these zones include albite, spodiimene, lepidolite, tourmaline, quartz, fluorite and columbite. Exceptional specimens found in pegmatitic environments are the result o f many factors, including extreme crystal fractionation, volatile increase, and stability. Crystal fractionation from the pegmatitic melt allows enrichment of rare-elements, such as Be and L i , that are essential elements in many gem minerals (e.g., beryl and spodumene). The volatile content of pegmatites also increases with fractionation and provides an environment with high element diffusion, facilitating large crystal sizes. The stability of the growth environment (chemical and structural) is important for the growth and preservation of delicate crystals. Magmat ic Syngenetic greisens and epigenetic veining associated with magmatic Sn-W-Mo mineralization are also common hosts for beryl (Figure 5). The Cordillera of western North America has been the subject of pegmatite and gemstone exploration and research for decades and was recently reappraised by Hart and Lewis (2000) and Legun (2005) with respect to Yukon and British Columbia, respectively. Exploration conditions considered in their reviews include restrictions to the Omineca Belt, intrusive ages of Cretaceous or younger, and rare-element and mineral associations as reported in the British Columbia and Yukon M T N F I L E and geochemistry databases. These studies also noted that productive intrusives are typically enriched in incompatible elements such as the Large-Ion-Lithofile elements (e.g., Rb, Cs), High-Field-Strength elements (e.g., Ta, Nb) and volatile elements (e.g., B , F). In British Columbia, well-documented aquamarine occurrences occur in the Bayonne magmatic Wall Zone (qtz) Figure 4. Schematic cartoon of a 'complex' pegmatite with commonly observed 'zones'. Abbreviations: alb = albite, brl = beryl, kspar = K-feldspar, qtz = quartz, spd = spodumene, tur = tourmaline (modified from Walton 2004). Figure 5. Schematic of relationships between an evolved granitic intrusion, greisen and epigenetic vein stockwork (modified after Blevin 1998 and Walton 2004). 7 arc (e.g., Brown 2004 and references therein) while in the more northern Cordillera an example of a well-mineralized W - M o greisen stockwork with aquamarine is Logtung (Nobie et al. 1984, Lowenstern and.Sinclair 1996, and Mihalynuk and Heaman 2001). The Cretaceous Tsa da Gl iza schist-hosted epigenetic beryl (var. emerald) occurrence in Yukon Territory also deserves note (Groat et al. 2003, Neufeld et al. 2004, Neufeld 2004). Granites with elevated L I L and volatile elements can also give rise to pegmatites, and granite 'fertility' with respect to pegmatite generation has been studied by Cerny (1991), Groat et al. (1995), and Trueman and Cerny (2002) and has been summarized by a number of authors (e.g., Keller 1990, Walton 2004). Metamorphic The southern European Alps are host to beryl occurrences some of which are interpreted to have metamorphic origins. Franz and Morteani (2002) review metamorphic mineralization in this area using two models: shear-hosted and vein-hosted ("Alpine Clefts or Vugs"). The shear-hosted emerald deposits of the region (e.g., Habachtal) have received the most attention due to their economic importance. These are environments where Be-rich rocks (e.g., metapegmatites) are juxtaposed against Cr-rich rocks (e.g., ultramafic rocks) by shear zones. Porphyroblastic beryl forms within the 'blackwall ' contact and precipitation of beryl is commonly described as syn- to post-deformation. A n example of a vein-hosted beryl occurrence located within the Tauern Window is the Fianel Fe -Mn deposit where quartz-dolomite-powellite veins with accessory blue beryl cut dolomite breccias within hematite-quartz-carbonate Fe -Mn ores (Brugger et al. 1998, Brugger and Giere 2000). Another interesting example of mineralization also related to Alpine metamorphism in the Tauern Window is the Schrammacher beryl occurrence (Franz et al. 1986). Here, blue beryl is reported to form as nodules replacing Be-bearing cordierite during amphibolite-facies metamorphism and contains up to 4 wt.% FeO. 8 C H A P T E R 2 R E G I O N A L A N D L O C A L G E O L O G Y Introduction The oldest rocks in the study area are mid-Proterozoic miogeoclinal sedimentary rocks that were deposited sub-parallel to the western margin of the North American craton (Templeman-Kluit 1976, Gabrielse and Yorath 1992). This dominantly shallow , water depositional environment was flanked to the east by deep-water sedimentation of the Selwyn Basin. Sedimentation continued on the Pelly Cassiar Platform until early Devonian and Mississippian when uplift and block faulting occurred (Templeman-Kluit 1976). Associated with these structural events was felsic volcanism on the platform and the deposition of chert pebble conglomerates within the Selwyn Basin. These sequences were then capped by calcareous argillite of mid-Triassic age. Sedimentation was succeeded by post-mid-Triassic east-north-east directed compressional tectonics and associated greenschist facies grade metamorphism (Templeman-Kluit 1976). It is these aforementioned events that dominantly gave rise to the present-day arrangement of the Pelly Mountains. The Tertiary-aged Tintina fault bounds the Pelly Mountains to the northeast (Templeman-Kluit 1979). To put the area into a larger context, the Cassiar Platform is part of the Omineca Belt and has been compared to the M c E v o y Platform (at the eastern margin o f Yukon-Tanana Terrane that is east of the Tintina Fault) and Kootenay Platform (southern British Columbia), while the Selwyn Basin has been compared to the deep-water sedimentary rocks of the intracontinental Kechika Trough in northern British Columbia (Gordey and Makepeace 2003). Figure 6 shows regional tectonic terrane maps of Yukon and Western Canada with relevant labels and features. Figure 7 shows a restored section across the Pelly Cassiar Platform and its adjacent tectonic elements as interpreted by Templeman-Klui t (1977), and Figure 8 is a cartoon of general Devono-Mississippian tectonic environment of northwestern Canada from Nelson et al. (2002). 9 Figure 6. Terrane map of Yukon (a) and more generalized terrane map of western Canada (b) with locations of Devono-Mississippian V M S deposits (modified from Came 2003 and Nelson et al. 2002, respectively). 10 OMINECA CRYSTALLINE BEIT YUKON CRYSTALLINE TERRANE PELLY - CASSIAR PLATFORM SELKYN BASIN TRIASSIC . . p t H H - v i W * |MISSISSIPPIA« •DEVONIAN Figure 7. Schematic of a restored cross section in the Quiet Lake map area (NTS 105F) from Templeman-Kluit (1976). Star indicates approximate stratigraphic location of alkaline volcanics of the Pelly Mountain Volcanic Belt and their intrusive syenite equivalents. Yukon-Tartana Terrane Ancestral North America AikaEk-hosted j£j£L MVF Mafic-hosled VHMS C i n n i p Pin* Point, Mafic-hosted VHMS fedk-hosted VHMS VHMS Homes take Y H U S R O H B Rnlayson lake belt RBI*V«MI lake belt ChuChua PeHyMtns. jgw* Y _ V _ y Y V Y Figure 8. Schematic of general Devono-Mississippian tectonics and metallogeny of the western Canadian Cordillera from Nelson et al. (2002). The synmetamorphic True Blue showing is hosted within alkalic intrusive and extrusive igneous rocks of the Cassiar Terrane. 11 Regional geology of the Pelly Mountains The Pelly Mountains primarily comprise Lower Paleozoic continental sedimentary rocks of the Cassiar Platform, Mississippian plutonic and volcanic rocks, and Cretaceous granitic rocks (Figure 9). The continental platform rocks were deposited in both deep and shallow water environments and consist of quartzite, chert, pebble conglomerate, siltstone, shale, limey shale, and carbonate (Gordey and Makepeace 2003). Mississippian strata include felsic volcanic rocks that intertongue with dominantly deep-water sedimentary rocks (Mortensen 1982). These volcanic rocks are commonly referred to as the Pelly Mountain Volcanic Belt ( P M V B ) and have been explored for Kuroko-type V M S mineralization (Hunt 2002). Syenite plugs, sills, and dikes are also present and are believed to represent the subvolcanic equivalent of the felsic volcanic rocks (Mortensen 1982). During emplacement of these igneous rocks into the Paleozoic stratigraphy they likely experienced alteration associated with locally developed hydrothermal cells (Hart et al. 2004). Mid-Cretaceous granitic to monzonitic plutons of the Cassiar Suite occur throughout the Pelly Mountains and range from small stocks to large batholiths and are thought to be of post-orogenic origin (Driver et al. 2000). The closest of these plutons lies 9 km to the southwest of the True Blue showing. The region was affected by three major deformational events (Mortensen 1982); F l and F2 are northwest trending and coaxial and are believed to have developed in response to regional thrusting, and F3 produced more localized northeast-trending warps and may be related to Cretaceous magmatic activity (Mortensen 1982). In a study local to the Ketza River mine, Fonseca (1997) divided events into four stages: D I and D2 produced large F l and F2 folds, D3 is documented as a thrusting event, and D4 is younger and related to mineralization likely of Cretaceous age. Regional fault patterns are dominantly caused by complex post-Late Triassic thrust faulting that gives a northwest-trending structural fabric. Four main thrust faults dip to the southwest and record an easterly to northeasterly transport direction. From northwest to southeast and structurally lowest to highest, these are the St. Cyr, Cloutier, Porcupine-Seagull, and McConnel l thrusts (Abbott 1986). These thrust faults cut units as young as the middle to Late Triassic Jones Lake Formation and are cut by middle Cretaceous intrusions, thereby bounding the age of the faults between the post Late-Triassic and middle Cretaceous 12 Figure 9. Simplified geological map of the Pelly Mountains within the Quiet Lake map sheet (NTS 105F) south of the Tintina Fault. Inset shows the general location within Yukon Territory. Teeth on thrust faults indicate upper plate. The rectangle corresponds to the area shown in Figure 10. Data plotted using UTM Zone 9N and NAD83. periods (Templeman-Kluit et al. 1976, Mortensen 1982). The age of the Jones Lake unit was determined using conodont biostratigraphy (Templeman et al. 1976) whereas various isotopic methods have been used to date the Cretaceous granitoids, as compiled in Breitsprecher et al. (2003). Middle greenschist to lower amphibolite mineral assemblages of almandine-biotite-chlorite-muscovite were noted in Mississippian pelites by Mortensen (1979) and were reported to have developed concurrently with regional deformation. Thus, regional metamorphic grade attained during regional compressional tectonics reached lower amphibolite facies to the southwest of the study area, but is more commonly middle greenschist facies throughout the Pelly Mountains (Tempelman-Kluit 1977, Chronic 1979, Mortensen 1982, Abbott 1986). Regional metallogeny of the Pelly Mountains Overview Timing of beryl mineralization within the regional geological setting was not immediately apparent from field relationships. Therefore, it is pertinent to discuss regional metallogeny and characteristics of these mineralizing events. Mineralization in the Pelly Mountains comprises two main temporal periods: Mississippian and Cretaceous. Epigenetic mineralization of uncertain origin in the Pelly Mountains has typically been attributed to Cretaceous magmatism. Mississippian Volcanogenic massive sulfide ( V M S ) showings and minor skarn showings related to Mississippian igneous activity have been previously studied by Mortensen (1979, 1982), Chronic (1979), Gordey (1977, 1981), and Hunt (1997, 2002). The volcanic rocks are commonly referred to as the Pelly Mountain Volcanic Belt ( P M V B ) and have been explored for base metal mineralization following the Kuroko-type V M S model. Hunt (2002) summarized major mineralized showings in the Pelly Mountains and described them as "primarily felsic volcaniclastic material with lesser felsic and minor intermediate sills, dikes, or flows" that have undergone pervasive carbonate, clay, and pyritic alteration. The syenitic subvolcanic equivalents of these rocks are thought to be 14 responsible for small R E E - U - T h enriched skarns and associated veining in the surrounding Paleozoic carbonates (Chronic 1979), but these styles of mineralization have yet to be conclusively dated. Cretaceous Epigenetic and skarn mineralization related to granitic igneous activity is of Cretaceous age and has been documented in the Pelly Mountains (e.g., Dick 1979, Abbott 1986, Cathro 1988, and Fonseca 1997). The most studied area with this type of mineralization is centered on the Ketza River A u deposit. There, Au-bearing veins and-mantos are thought to be associated with Cretaceous (-108 Ma) granitic intrusions (Abbott 1986, Fonseca 1998). Distally associated with these A u mineralized areas are abundant Pb-Ag quartz-carbonate veins (Cathro 1988, Fonseca 1998). Cretaceous W ± M o ± S n skarns are also common in the Pelly Mountains, the most significant perhaps being the Stormy deposit adjacent to a mid-Cretaceous pluton (Dick 1979). Loca l setting at the True Blue showing The Shark property The Shark property is located 50 km south of Ross River in southern Yukon. The property is approximately 10 km southwest of the former Ketza River mine and its gravel airstrip. Year-round access to the Shark claims is possible via helicopter from Ross River. A gravel road from the Robert Campbell Highway to the Ketza River mine site is usable during the summer and fall. A four-by-four trail extends from the mine to the northern part of the Shark property. During the 2003 and 2004 field seasons, access to the property was by truck to the Ketza River airstrip, and then from the airstrip to the property via helicopter. The property is located within the Pelly Mountains on the southwest side of the Tintina fault. It is immediately north of White Creek within the headwaters of the McConnel l River. Local terrain consists of rugged mountains separated by wide glaciated valleys with fairly gentle floors. Valley bottoms are mostly covered by glaciofluvial outwash and are flanked by lateral moraines and moderate to steep hillsides (typically 20 to 50°). The property is centred on a prominent west trending ridge with a 15 series of north trending spurs. Outcrop is most abundant in cirques on the north side of the main ridge and in actively eroding creek cuts. Ice sheets covered the entire Pelly Mountain area during the Pleistocene (Bond and Kennedy 2004) and alpine glacial features such as cirques, tarn lakes and moraines are common. Elevations on the property are between 1250 and 2150 m. Treeline is at about 1500 m. Vegetation ranges from scattered stunted spruce, balsam and wi l low at lower elevations giving way to buckbrush and moss and ultimately to lichen-covered rock at higher elevations. The climate in the Pelly Mountains is characterized by very cold, long winters, truncated fall and spring seasons and short, cool summers. The normal operating season for mineral exploration is from late M a y through early October. W o r k history A s noted, the Pelly Mountains have received attention from geologists i n industry, government and academia. Since the late 1960s considerable work has been done in the Ketza-Seagull District and those that are in the immediate vicinity of the True Blue showing are described briefly. In 1976, the Guano claims were staked by Ukon Joint Venture (Chevron Minerals Limited and Kerr Addison Mines Limited) to cover the eastern portion of the present Shark property. Those claims were explored for uranium and rare earth elements (REE) associated with skarns and veins developed peripheral to a Mississippian syenite stock (Chronic 1979). Concurrent to the exploration program, F. Chronic completed a M . S c . study of the U - R E E mineralization at the Guano claims under the supervision of Dr. C. Godwin at the University d f British Columbia in 1979. Her thesis included petrographic investigations of the mineralized zones and host rock stratigraphy, and geochronological investigations as well as minor geochemistry of relevant rocks. In the late 1980s the White and PS claims were staked by Mountain Province Min ing Inc. to cover a large gold target. Most of those claims were north of the Shark claims but some covered the eastern portion of the current Shark property (Deklerk and Traynor, 2004). In 1988, B . Ha l l staked the Matthew claims, which included what is now the southwestern corner of the Shark property, in order to cover a Kuroko-type V M S target. After a number of option agreements, the Matthew claims expired during the 16 1990s and were re-staked as the Mamu-Bravo-Kulan claims (Deklerk and Traynor, 2004). , In winter of 2002 the Shark claims were staked for True North Gems Inc. by Archer, Cathro & Associates (1981) Ltd. (Archer Cathro) during a regional gemstone exploration program. This staking was prompted by data collected by D . Eaton of Archer Cathro during the 1976 Ukon Joint Venture program. Eaton's field notes described an unidentified blue mineral within a quartz vein that cut a syenite boulder. This blue mineral was later identified as beryl by Prof. L . A . Groat shortly before staking occurred. F ie ldwork Geological fieldwork during the 2003 and 2004 field season comprised mapping, prospecting, soil sampling, silt sampling, rock sampling, specimen extraction and claim staking. Archer Cathro conducted the programs during both years and academic work was carried out concurrently with exploration work. Crew size varied between 3 and 4 persons and fly-camp durations were 2 and 4 weeks during the 2003 and 2004 programs, respectively. One two-week visit was paid to the property by a crew o f 4 persons prior to the author's involvement. The first half of the 2003 program consisted of preliminary mapping and prospecting in order to delineate beryl-bearing zones. Shark B o w l , G i l l Zone, F in Zone and Guano Ridge (Figure 2) were designated high priority areas with the three former areas hosting beryl discoveries and the last area having prospective geology. The second half of the program focused on the extraction of beryl-bearing veins from Shark Bowl . A diamond disc hand saw was used to extract material during the 2003 field season; however this only allowed incisions up to -12 cm deep. A s a result, quartz vein extraction within Shark B o w l during the 2003 field season focused on beryl mineralization on exposed vein surfaces. Although gem grade material was obtained in 2003, some of the crystals had suffered damage from natural surface processes such as rock fall. Over 100 occurrences of beryl were documented, of which 24 were selected for extraction, and two boulders (-115 and -60 kg) were selected for mini bulk sampling. Exploration,during the 2004 field season focused on Shark B o w l and Guano Ridge. A s determined in 2003, Shark B o w l was the main area of dark blue gem beryl 17 mineralization and Guano Ridge showed promising geological and geochemical characteristics reminiscent of Colombian-type emerald deposits (Walton 2004). Two weeks were spent on the eastern portion of the property, and mapping, prospecting, silt sampling and soil sampling were carried out. The fly camp was then relocated to Shark B o w l where the focus of the program was on extraction of undamaged quartz veins. Several improvements were made to the extraction process through utilizing two 18-inch (~ 40 cm) diamond chain saws. To cool the saws three pumps, dril l hose, and a number of holding drums were flown into the head of the Shark B o w l via helicopter to prepared sites at approximately 70-m vertical intervals. A t each pump and active cutting sites 170-litre drums were used to hold water and supply head to the saws via smaller garden hoses. More than 100 additional occurrences of beryl were discovered and a total of 108 specimen extractions were executed. In total, 785 kg of concentrated vein material was removed from syenite talus and bedrock exposures in Shark B o w l during the two-week period. Geological unit descriptions The geology of the Shark claims (~4 x ~5.5 km) is dominated by the southeast corner of an elongate, northwest-southeast trending, ~36 k m 2 Mississippian syenite pluton (unit M y ) (Figures 9,10, and 11). Structurally above and spatially to the south of the syenite are its extrusive equivalents, the felsic volcanic rocks of the Pelly Mountain Volcanic Belt (unit Mva) . To the east, the syenite is in fault contact with Palaeozoic carbonate rocks and shale of the Cassiar Platform. From the fault contact with the syenite eastward (youngest to oldest) they consist of units u D M s , SDc, and uCOs. Sedimentary rocks Unit u D M s consists of thinly laminated, dark grey to black shale to phyllite that weathers to blocky cobble- and smaller-sized clasts. Quartz veins in this unit are rare, barren, moderately deformed, and up to 4 cm in width. Minor euhedral pyrite crystals are present in this unit. Unit SDc includes thinly to thickly bedded grey limestone and orange-weathering dolomite with minor quartzite. This Siluro-Devonian unit forms the 18 topographic contour (100 m interval) approximate claim boundary limit of Quaternary overburden Quaternary • Qob - overburden Mississippian • My - syenite • Mva - metavolcanic rock Upper Devonian to Mississippian • uDMs - shale Silurian to Devonian • SDc - carbonate Upper Cambrian to Ordovician D uCOs - shale, limestone Figure 10. Geological map of the True Blue showing within the Shark claim block, Southern Yukon Territory. Stars indicate zones of beryl mineralization and triangle indicates REE mineralization. Contours and data plotted using UTM Zone 9N and NAD 27. Figure 11. Photographs and mosaics from the True Blue showing and Shark property, (a) is looking south into Shark Bowl: Stars indicate major beryl locations and circles indicate locations of water pumps, (b) is looking north at the fault contact between the dark coloured unit uCOs and the light coloured unit SDc on the east side of Guano Ridge, (c) and (d) show the typical morphology of sigmoidal tension gashes cutting syenite at the True Blue showing. Fields of view are approximately 1 m and 15 cm for (c) and (d), respectively. 20 bulk of "Guano Ridge" which hosts subeconomic R E E and U mineralization (Chronic 1979). Minor quartz-calcite veining is present within this unit and fault gouge marks the contact with the Late Cambrian to Ordovician uCOs unit. Unit uCOs comprises grey to black lustrous phyllite and minor black shale which are typically thinly bedded and moderately deformed. Quartz-calcite veining in the unit is deformed and transposed parallel to bedding. Igneous rocks Mississippian extrusive igneous rocks (unit Mva) are abundant at the southern edge of the property and consist of felsic metavolcanic rocks that comprise lapil l i tuffs, volcanic breccias, trachyitic flows, minor sedimentary interbeds of chert and phyllite, and sericite and chlorite-talc altered schist (Templeman-Kluit et al. 1976, Templeman-Kluit 1977, Mortensen 1982, Hunt 2002). This unit typically ranges from pale green to grey to maroon in colour, and weathers to form platy to blocky talus. Quartz veins hosted in unit M v a typically contain siderite and rare fluorite and sulfide minerals. The 12 k m long, 3 km wide light grey to pink to dark green syenite stock (unit M y ) is equigranular and medium- to fine-grained in texture. The syenite forms prominent cliffs along ridges due to its resistant weathering. Xenoliths of unit M v a , too small to be mapped at a regional scale, are present within the syenite, are more abundant at higher elevations, and may represent roof pendants. Quartz veins are hosted within the syenite by sigmoidal tension gashes (Figure 11) and are more abundant near contacts with the metavolcanic rocks. They range in size from mm- to m-scale and accessory minerals include siderite, ankerite, allanite, fluorite, and minor albite, various sulfide minerals, and Fe-Ti-Nb oxide minerals. They are described more fully in Chapter 5. Although Paleozoic metasedimentary and metavolcanic rocks in the region have undergone ductile deformation, the Mississippian syenite within the study area shows no foliation in outcrop. However, feldspars from the pluton at the True Blue showing do show signs of brittle deformation in thin section and abundant tension gashes are present near the upper contact of the syenite with the Mississippian strata. This suggests that the metavolcanic rocks and the syenite had different rheological responses to the regional deformation events. 21 C H A P T E R 3 G E O C H E M I S T R Y OF T H E S Y E N I T E H O S T R O C K S Whole-rock geochemistry Major and trace elements were obtained from eight whole-rock analyses of syenite. Samples were collected from within Shark B o w l and were chosen on the basis of degree of alteration and variation within the main cirque. A l l samples were trimmed to remove alteration rinds attributed to surface weathering. The data are summarized in Table 1 and analytical procedures and uncertainties are given in Appendix 1. Six samples of the syenite were analyzed during previous studies by Chronic (1979) and Mortensen (1979) and are summarized in Table 2. The two samples collected by Mortensen (1979) originated from an outcrop across the McConnel l River valley to the northwest of the True Blue showing and the four samples from Chronic (1979) originated from the eastern extent of the syenite within the Shark property. M a j o r elements S1O2 concentrations for the syenite range from 58.70 to 63.70 wt.% and average -61.5 wt.%. N a 2 0 + K 2 0 values range from 7.78 to 11.26 wt.% and average -10 wt.%, while Na20/K20 values average - 2 0 and attain 79. CaO values are low, ranging from 1.39 to 3.74 wt.% and averaging 2.48 wt.%. M g O values are also low, ranging from 0.369 to 1.54 wt.% and averaging 0.79 wt.%. Fe203 values averaged 4.73 wt.% while FeO analyses averaged 2.78 wt.%. F e / M g + Fe values range from 0.70 to 0.93 and average -0.85 and Ti02 values average 0.59 wt.% and attain 0.79 wt.%. Cross, Iddings, Pirsson and Washington normative (CIPW norm) calculations indicate that the least-altered samples with elevated K2O are silica undersaturated to slightly quartz normative (0 - - 1 0 % quartz) and contain - 8 0 % modal feldspar where orthoclase > albite » anorthite (Table 3). Average Na 2 0+K20, Si02 and CaO values from previously analyzed samples (-11, -63 , and -1.5 wt.%, respectively) are similar to new analyses and also indicate that 22 T A B L E 1. S E L E C T E D W H O L E - R O C K G E O C H E M I S T R Y OF S Y E N I T E F R O M T H E T R U E B L U E S H O W I N G DTTH01 D T T H 0 2 D T T H 1 l c D T T H 0 6 THIII02 D T W R 0 1 P2O5 (wt. %) 0.08 0.04 0.06 0.07 0.02 0.07 S i 0 2 61.00 63.70 60.43 58.70 59.87 63.02 T i 0 2 0.65 0.50 0.79 0.62 0.64 0.57 A1203 17.60 15.47 17.16 16.56 16.01 15.95 C r 2 0 3 0.01 O . 0 1 <0.01 O . 0 1 0.01 0.01 F e 2 0 3 5.73 2.65 4.50 7.39 6.04 3.90 M g O 0.79 1.14 0.51 1.54 0.46 0.84 CaO 2.12 3.74 2.91 2.96 1.39 2.35 M n O . 0.06 0.02 0.02 0.07 0.16 0.07 SrO 0.04 0.01 0.02 0.02 <0.01 O . 0 1 BaO 0.01 0.08 0.01 0.02 0.10 0.02 N a 2 0 8.69 5.41 8.13 7.33 3.90 8.29 K 2 0 0.11 3.05 033 0.45 7.36 0.40 L O I 2.78 4.19 3.78 4.13 3.70 4.26 Total 99.68 99.99 98.64 99.84 99.67 9975 A g (ppm) <1 <1 • <1 <1 <1 <1 B 40 <20 <20 <20 <20 30 B a 46.2 695 35.4 83.5 912 36.9 Be 5.47 11.75 4.46 6.42 . 9.36 4.42 Ce 298 568 235 343 305 174 C l 160 280 270 270 330 150 Co 1.3 1.6 0.5 0.9 1.2 1.1 Cr 120 90 60 40 120 80 Cs 0.2 0.7 0.4 1.3 0.5 0.3 C u 9 <5 <5 <5 11 7 D y 25.7 24.1 17.7 17.3 13.8 9.3 Er 16 16.2 10.8 10.6 8.9 5.6 E u 2.4 1.3 1 1.1 0.2 1.3 F 2990 5950 2980 2590 2010 720 Ga 40 37 36 39 35 32 G d 27.4 29.5 16.6 21.8 17.7 10.6 H f 44 45 29 26 21 17 Ho 5.9 5 3.5 3.5 2.8 1.6 L a 163.5 309 122.5 183 157.5 84 23 L i 5.5 10.8 2.3 18.6 3.8 4.8 L u 2.1 2.5 1.6 1.6 1.2 0.7 M o 3 4 23 3 9 .3 Nb 546 435 351 308 247 198 N d 124 173.5 79.1 122.5 102.5 60.2 N i 8 5 5 5 7 5 Pb <5 6 5 <5 6 <5 Pr 36.5 55.2 24 35.7 32 • 18.4 Rb 3.4 106 10.4 23.4 138.5 14.5 Sc 1.6 2 2.4 2.2 1.6 2 Sm 21.3 29.6 15.3 22 19 11.7 Sn 8 22 5 15 13 6 Sr 53.3 58.3 78.1 63.4 56.9 68.4 Ta 29.9 28 18.7 14.7 13 9.7 Tb 5 4.3 2.9 3.2 2.6 1.5 Th 111 121 49 51 49 25 T l <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 T m 2.4 2.5 1.6 1.5 1.2 0.7 U 14.2 18 10.5 9.9 9.5 5.8 V <5 <5 <5 <5 <5 <5 w .6 6 6 4 8 3 Y 169 131 98.4 99.5 83.6 50.8 Y b 14.2 17.1 10.5 10 8.1 5.4 Z n 42 19 22 54 42 15 Zr 1890 1715 1170 1085 962 693 R E E + Y 913.4 1368.8 640.5 876.3 756.1 435.8 Eu/Eu* 0.30 0.13 0.19 0.15 0.03 0.36 FeO % 4.06 1.09 2.38 4.89 4.14 2.44 C 0 2 % 1.4 3.3 3.4 3.6 3.1 3.6 C % 0.39 0.91 0.91 0.97 0.85 0.98 H 2 0 % 0.02 0.08 0.07 0.07 0.11 0.03 H 2 0 + % 0.98 0.53 0.5 0.91 0.46 0.46 Note: Maximum values are shown in bold and analytical procedures are described Appendix 1. 24 T A B L E 2. W H O L E - R O C K G E O C H E M I S T R Y OF P R E V I O U S L Y A N A L Y Z E D S Y E N I T E N E A R T H E T R U E B L U E S H O W I N G 860* Syenite 858* Quartz keratophyre n38 T Unit 16, Syenite n5t-Unit 16, Syenite n6 T Unit 16, Syenite n l 0 T Unit 16, Syenite P 2 0 5 (wt. %) 0.10 0.10 0.14 0.14 0.16 0.13 S i 0 2 64.70 72.30 60.47 61.69 61.92 60.13 T i 0 2 0.57 0.22 0.60 0.56 0.53 0.66 AI2O3 19.80 16.90 17.58 18.24 15.98 16.30 F e 2 0 3 0.90 0.80 6.53 2.81 7.52 8.54 M g O 0.20 0.20 0.99 0.67 0.85 1.03 CaO 1.10 0.10 1.50 3.91 1.37 1.47 M n O 0.00 0.00 0.07 0.05 0.08 0.09 N a 2 0 7.70 10.80 3.74 5.78 5.05 3.60 K 2 0 4.60 0.10 7.99 5.93 4.69 6.53 L O I 0.50, 0.10 0.61 0.54 0.87 1.43 Total 100.Q7 101.52 100.08 100.18 98.86 99.78 B a (ppm) 4113 38 Nb 120 331 Rb 66 5 Sr 376 38 Y 39 118 Zr 349 2094 Ce 180' 120 214 165 D y 6.7 4.8 9.6 6 E u 1.45 1.48 1.8 1.8 L a 66 39 47.5 64 L u 0.56 0.42 0.61 0.49 N d ' 84 69 120 48 • Sm 8.03 7.06 7.42 8.94 Tb - 0.79 0.6 1.2 0.8 Y b 4.4 3.4 4 3.6 R E E Total 351.9 245.76 406.1 298.6 *From Mortensen (1979). r From Chronic (1979). 25 T A B L E 3. C I P W N O R M A L I Z A T I O N F O R S Y E N I T E S A M P L E S D T D T - D T - D T - THII I D T TH DT-TH01 T H - T H - T H - 02 W R III02* TH-02 11c 06 01 02* Quartz 2.49 12.98 5.45 3.87 3.51 7.37 11.47 19.97 Corundum 6.33 4.21 Orthoclase 0.65 18.02 1.95 2.66 43.49 2.36 43.49 18.02 Albite 73.53 45.78 68.79 62.02 33.00 70.15 33.00 45.78 Anorthite 8.69 8.92 9.35 10.95 4.44 5.13 -12.83 -2.57 Nepheline Diopside 1.11 6.14 3.89 2.75 2.01 5.06 Hypersthene 6.51 0.69 8.47 5.37 2.04 6.42 2.84 Olivine Magnetite 2.42 2.13 3.07 3.62 2.75 •2.12 2.75 2.13 Ilmenite 1.23 0.95 1.50 1.18 1.22 1.08 1.22 0.95 Apatite 0.19 0.09 0.14 0.16 0.05 0.16 0.05 0.09 Acmite Leucite K-Metasilicate Na-Metasilicate CaDiSilicate Kaliophilite Wollastonite 0.63 Hematite 0.09 0.09 Titanite Rutile Perovskite Calcite 7.05 7.51 Agpaitic Index 0.82 0.79 0.80 0.76 0.90 0.88 0.90 0.79 M g # 24.10 58.51 24.14 33.49 15.22 35.10 15.22 58.51 Note: * indicates sample was normalized with C 0 2 values included. 26 T A B L E 4. C I P W N O R M A L I Z A T I O N F O R H I S T O R I C A L S Y E N I T E S A M P L E S 858* 860* n38 T 115+ n6 T n l 0 T Quartz 10.84 3.84 10.84 9.88 Corundum 0.06 0.39 0.39 0.49 0.95 Orthoclase 0.59 27.18 47.22 35.04 27.72 38.59 Albite 86.37 65.15 31.65 47.60 42.73 30.46 Anorthite -0.16 4.80 6.53 6.31 5.75 6.44 Nepheline 0.71 Diopside. 3.61 Hypersthene 0.50 0.28 2.47 2.12 2.57 Olivine 0.15 Magnetite Ilmenite 0.15 0.11 0.17 0.19 Apatite 0.23 0.23 0.32 0.32 0.37 0.30 Acmite 2.31 Leucite K-Metasilicate Na-Metasilicate 0.56 CaDiSilicate Kaliophilite Wollastonite 2.42 Hematite 0.90 6.53 2.81 7.52 8.54 Titanite Rutile 0.22 0.57 0.52 0.44 0.56 Perovskite 0.86 Calcite Agpaitic Index 1.06 0.89 0.84 0.87 0.84 0.80 M g # 66.46 63.78 54.57 65.39 47.25 48.87 *From Mortensen (1979). r From Chronic (1979). 27 the syenite is alkaline in character. These previously analyzed samples show variable N a 2 0 / K 2 0 values from 0.46 to 108, with three samples near 1. FeO values average -4.5 wt.%, Fe /Mg + Fe values range from 0.80 to 0.90 and average -0.85, and T i 0 2 values average 0.52 wt.% and attain 0.66 wt.%. C I P W norm calculations indicate these samples are quartz normative with albite equal to or much greater than orthoclase (Table 4). Trace elements In general, the syenite is enriched in F (to 5950 ppm), R E E (total to 1237 ppm), and H F S E (Zr to 1890 ppm, Nb to 546 ppm, H f to 45 ppm, and Ta to 29.9 ppm), and moderately enriched in Be (to 11.75 ppm). Boron is barely detectable in three samples (-30 ppm) but the remaining samples are below analytical detection. L i concentration is variable, ranging from 1.8 to 18.6 ppm. Both W and M o are present in low abundance, generally below 10 ppm, while Sn concentrations average -11 ppm. Cr contents are also low, averaging 80 ppm and attaining 120 ppm. Rb values range from 3.4 ppm to 138.5 ppm but cluster around -15 ppm. Eu/Eu* ( E u N / [ V G d N x S n i N ] ) values range from 0.03 in the sample with the largest negative anomaly to 0.36 in the sample with the smallest negative anomaly. L a N / S m N values average 5.25 and G d N / L u N values average 1.59, while Latv|/LuN values average 11.11. Trace elements analyzed by Mortensen (1979) include Ba , Nb , Sr, Y , Zr, V and Cr. A l l element concentrations are similar to the newer values with the exception of elevated B a (4113 ppm) and Sr (376 ppm) in sample "860". A n incomplete sequence of rare earth elements are given by Chronic (1979). These values are typically lower than data collected in this study and missing elements in the R E E series (e.g., Gd) makes comparison and interpretation difficult. Interpretation of whole-rock geochemistry Nomenclature and classification of igneous rocks is commonly assisted by major element analyses and the display of this data in well known diagrams. These diagrams have been constructed using well-characterized and typically unaltered samples, thus requiring similar conditions for plotting other data. The samples analyzed in this study have been variably altered, and thus the interpretation of the diagrams must be 28 approached with some caution. However, most of the diagrams suggest interpretations that are consistent with non-geochemical observations and interpretations. C I P W norm calculations with CO2 values removed suggest the pluton is syenite to quartz-syenite in character, with normative quartz ranging from 0 to - 14% and averaging - 7 %. C I P W norm calculations with CO2 values included result in unlikely mineral assemblages that are normative in corundum (-7%), quartz (-25%) and Na-carbonate (-6%). These results suggest that the additional C 0 2 is not of primary igneous origin, but instead was acquired during a later alteration event. Na20 + K2O and S i 0 2 concentrations (-10 and -62 wt.%, respectively) indicate that the syenite is alkaline in character and the Shand Index diagram (Figure 12) shows all samples as peraluminous. N a 2 0 / K 2 0 values are variable, ranging from 0.5 to 108, indicating both Na-dominant and K-dominant samples. Figures 13a and 13b show K2O versus Na20 and Rb versus Na20. Figure 13a suggests that alkali has occurred exchange (Na replacing K ) , which is consistent with petrographic observations of K-feldspar albitization. Accordingly, a similar correlation is seen between N a 2 0 and Rb (Figure 13b), suggesting that this trace element is being mobilized concurrently with K from K -feldspar. B a enrichment is well correlated with concentration of K2O, while Sr enrichment is weakly correlated with concentration of Na20. Discrimination diagrams using Y , Nb , Y b , and Ta (Figures 14a, b) place the syenite in the "within-plate granite" (WPG) tectonic setting field (after Pearce et al. 1984). Discrimination diagrams utilizing Rb result in conflicting classification of the syenite, which supports the mobility of Rb (Figure 14c). The R E E spidergram (Figure 15a) and R E E ratios normalized to chondrite (CI values from McDonough and Sun 1995) indicate that the syenite is light rare earth element ( L R E E ) enriched, shows a pronounced negative E u anomaly (Eu/Eu* -0.19), and has a flat heavy rare earth element (HREE) signature. The trace element diagram (Figure 15b) indicates that the syenite is relatively depleted in Cs, Rb, Sr, Ba , and Pb. It also shows that there is general enrichment in U , 29 0.5 0.7 0.9 1.5 1.7 1.9 1.1 1.3 AI 20 3/(CaO+Na 20+K 20) Figure 12. Shand Index diagram of syenite from the True Blue showing (after Maniar and Piccoli 1989). Primary O This Study X Chronic (1979) A Mortensen (1979)1 Secondary 6 8 Na 20 (wt. %) 10 12 160 140 120 1" 100 Q. £ 80 . 2 * 60 40 -i 20 0 Primary O This Study A Mortensen (1979) Secondary o .0° 6 8 10 Na 20 (wt. %) 12 Figure 13. Plots of Na 20 versus K 2 0 (a) and Rb (b). The negative correlations are suggest progressive degrees of alteration (albitization). 30 100 a ' oo 0.1 0.1 1 10 Yb(ppm) 10000 1000 E a. a. '100 10 WPG VAG + syn-COLG o This Study A Mortensen (1979) 10 100 1000 E Q. a A DC 100 10 100 1000 Y (ppm) -* C syn-COLG o A WPG VAG / o ° o A o This Study O ORG A Mortensen (1979) 10 1000 100 Y+Nb (ppm) Figure 14. (a) Ta-Yb, (b) Nb-Y and (c) Rb-Y+Nb tectonic discrimination diagrams (after Pearce et al. 1984) for syenite from the True Blue showing and elsewhere in the Pelly Mountains. O R G = ocean-granite; syn-COLG = syn-collisional granite; V A G = volcanic-arc granite; W P G = within-plate granite. 31 10000 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 10000 1 I. I Cs Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Er Yb Y Lu Figure 15. Chondrite-normalized rare-earth element (REE) and trace-element diagrams from selected syenite whole rock data. Normalization values from McDonough & Sun (1995). 32 Th, Nb , and Ta but minor variable behaviour of U and Th. The overall consistent behavior of the R E E and trace-element profiles between samples suggests that alteration has not had a profound selective impact on the patterns. Consequently, the negative E u anomaly is interpreted as evidence for fractional crystallization of plagioclase and the suppression of H R E E could be attributed to the presence of garnet in the source region. Plagioclase fractionation would also have an impact on the Sr concentrations since it is a common host mineral for this element. The varying depletions of Rb, Ba , and Pb suggest that these elements may have been mobilized out of the system during alteration. This correlates well with ubiquitous albitization of K-feldspar, in which Rb, Ba , and Pb can commonly substitute for K . In addition, the samples that show the highest concentrations of Rb and B a also have the highest concentrations of K 2 0 and exhibit less overall albitization in thin sections. Petrogenesis The syenite is believed to be a high-level intrusion emplaced into undeformed Paleozoic marine carbonate and clastic sediments of the Cassiar Platform in an extensional environment. Evidence for this includes the appearance and preservation of extensive coeval volcanic rocks, the development of Kuroko-type V M S showings, miarolitic cavities within the syenite, local sedimentary breccias, and injection of syenite sills within the volcanic succession (Tempelman-Kluit 1977, Mortensen 1979, Doherty 1997, Hunt 2002). A s noted, the syenite is alkaline in composition and discrimination diagrams suggest it has an A-type composition. This type of magma is commonly interpreted to have been generated through assimilation-fractional crystallization processes of a mantle-sourced partial melt, but crustal and mixed sources have also been postulated (Whalen et al. 1987, Eby 1990, 1992, Creaseref a/. 1991, Turner et al. 1992, Mushkinef al. 2003). Fractional crystallization has been interpreted from the R E E diagram and like many alkaline intrusives with A-type characteristics, the syenite at the True Blue showing is enriched in R E E s and HFSEs . Because pervasive alteration of the syenite has destroyed much of the primary mineral assemblage (see Chapter 4) and mobilized at least some 33 elements (i.e. K and Rb), conclusive statements and calculations regarding the petrogenesis of the syenite are difficult to make. 34 C H A P T E R 4 M I N E R A L O G Y OF T H E S Y E N I T E H O S T R O C K Introduction Syenite is defined as igneous rock comprising essential alkali feldspars (at least 65%) with minor to no quartz (less than 10%) and commonly includes alkali-bearing ferromagnesian minerals. A review of nomenclature is given by Mitchell (1995); however the extent of alteration of the syenite at the True Blue occurrence hinders further genetic classification. Moderate to strong alteration affected the syenite at the True Blue showing. It is characterized by albitization, carbonation, and likely by the addition of iron in the form of carbonate and simple oxides and sulfides. Primary minerals have mostly been destroyed, leaving only uncommon relict K-feldspar cores and altered zircon. The current alteration minerals present consist of albite, chlorite, biotite, Fe-Ca-Mg carbonates, and minor oxides, quartz, fluorite, REE-phosphates and REE-carbonates. Primary minerals Alkali feldspars Potassium-feldspar occurs as a primary phase which has been replaced by albite. Degree of replacement is commonly complete, indicating pervasive sodium metasomatism. Laths are up to several cm in length, but K-feldspar is most commonly seen as cores within altered phenocrysts (Figure 16). The K-feldspar crystals commonly exhibit Carlsbad twins and in rare instances phenocrysts are broken. Electron microprobe analyses ( E M P A ) show that these feldspar crystals have an average composition of OrgyAbo3Ano (Table 5). 35 Figure 16. Thin section photomicrographs (a, b, d) and B S E image (c) of altered and deformed feldspar within syenite at the True Blue showing, 'a' (XPL), 'b' (XPL), and 'c' (BSE) all show the same region, in which primary twinned K-feldspar has been partially replaced by albite. The core of K-feldspar visible in the centre of these images measures approximately 400 urn in diameter. In 'c', K-feldspar is indicated by the lighter tone (higher average atomic mass), while replacement albite has a darker tone (lower average atomic mass). In'd' (XPL), biotite + ankerite + Fe-oxide are seen along fractures in Carlsbad-twinned K-feldspar in proximity to alteration albite. The field of view is approximately 1.5 cm. 36 T A B L E 5. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F F E L D S P A R F R O M T H E T R U E B L U E S H O W I N G . Sample D T T H D T T H D T T H T H V - T H V - D T T H 02-2-20 02-1-7 02-5-23 01-5-3 01-3-1 02-1-6 S i 0 2 (wt.%) 65.27 64.17 72.61 68.80 69.60 68.17 AI2O3 18.42 18.48 17.30 19.72 19.52 19.43 M g O 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.02 0.03 0.09 0.01 0.04 M n O 0.00 0.00 0.00 0.03 0.00 0.00 FeO 0.00 0.05 0.02 0.00 0.27 0.09 N a 2 0 0.46 0.20 10.49 11.62 11.70 10.21 K 2 0 16.33 16.76 0.06 0.07 0.00 2.42 Total 100.49 99.67 100.50 100.33 101.10 100.36 S i 4 + (apfu) 3.002 2.985 3.122 2.993 3.005 2.992 A l 3 + 0.998 1.013 0.877 1.011 0.993 1.005 M g 2 + 0.000 0.000 0.000 0.000 0.000 0.000 C a 2 + 0.000 0.001 0.001 0.004 0.000 0.002 M n 2 + 0.000 0.000 0.000 0.001 0.000 0.000 F e 2 + 0.000 0.002 0.001 0.000 0.010 0.003 N a + 0.041 0.018 0.874 0.980 0.979 0.869 K + - 0.958 0.995 0.003 0.004 0.000 0.136 Or (mol.%) 95.90 98.13 0.34 0.40 0.00 13.51 A n 0.00 0.10 0.11 0.40 0.00 0.20 A b 4.10 1.78 99.54 99.19 100.00 86.30 Note: The following standards were used: orthoclase (S iKa , XJCa), anorthite ( A l X a , CaJTa), diopside (MgKrx), rhodonite (MnATa), fayalite (FeKa), albite (Na^a). Compositions were recalculated on the basis of 8 O apfu. Maximum values are shown in bold. P, Sr, Cs, and B a were sought but not detected. 37 Zircon Zircon occurs as a primary mineral and the average size of the euhedral to subhedral crystals is 300 * 150 um. The crystals are typically dark coloured, translucent, and locally fractured. The zircon crystals are rarely compositionally zoned and uncommonly exhibit cores. The crystals are commonly surrounded by secondary carbonate, oxide, and phyllosilicate minerals (Figure 17). Acceptable E M P A analyses (between 98.5 and 101.5 wt.%) of zircon average ~100.4 wt.%, whereas five samples exhibited high totals (101.92 wt.% and greater) (Table 6). Analyses from the population with high totals generally show results with little substitution for Si and Zr, while those in the acceptable range show more variation in chemical content. Phosphorous was sought using S E M - E D S and during an initial stage of the E M P A work, however it was not detected. The analyses with acceptable totals can be divided into two populations based on Fe content. Three samples belong to the higher group (FeO > ~3.5 wt.%) whereas 10 belong to the lower group. H f 0 2 concentrations across the entire population range from 0.76 to 2 wt.% and average 1.195 wt.%. Figure 18 shows Zr, Si , and A l plotted against Fe (all in apfu). The negative correlations between Fe and Zr, and Fe and Si suggest that either Fe is substituting into the Zr site, or it is a function of alteration. Supporting the idea of alteration is the rough positive correlation between Fe and A l . One sample in the high Fe population also contains appreciable amounts of CaO (1.29 wt.%), further suggesting a connection with alteration. Another possibility for site assignment of Fe, A l and Ca is into an interstitial site, as suggested by Hoskin et al. (2000). Although this may work for samples with detectable, but low, FeO, it is unlikely to be the most probable mechanism for explaining the ~5 wt.% FeO (~0.5 apfu) in this study. 38 Figure 17. Photomicrographs in X P L (a) and PPL (b) of altered and fractured zircon in syenite. Fractures are filled with mainly fluorite and lesser rare-element silicates, fluoro-carbonate and oxide minerals. 39 T A B L E 6. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S O F Z I R C O N F R O M T H E T R U E B L U E S H O W I N G TH02-5-14TH02-B-6 TH02-B-5 TH02-5-18TH02-B-3 TH02-5-8* Z r 0 2 (wt.%) 66.82 64.19 64.16 66.29 58.88 67.18 H f 0 2 0.84 2.00 1.76 0.79 1.63 0.78 T h 0 2 0.00 0.00 0.20 0.00 0.21 0.00 U 0 2 0.00 0.00 0.00 0.00 0.00 0.00 S i 0 2 33.72 33.33 33.46 34 .14 30.66 33.97 A 1 2 0 3 0.00 0.04 0.03 0.00 0.14 0.00 CaO 0.00 0.08 0.00 0.00 1.29 0.00 FeO . 0.00 0.58 0.15 0.12 . 6.69 0.13 Total 101.38 100.22 99.76 101.34 99.50 102.06 Z r 4 + (apfu) 3.917 3.822 3.828 3.875 3.617 3.911 Hf 4 " 0.029 0.070 0.061 0.027 0.059 0.027 T h 4 + 0.000 0.000 0.006 0.000 0.006 0.000 U 4 + 0.000 0.000 0.000 0.000 0.000 0.000 S i 4 + 4.054 4.070 0.004 4.092 3.863 4.056 A l 3 + 0.000 0.006 4.094 0.000 0.021 0.000 C a 2 + 0.000 0.010 0.000 0.000 0.174 0.000 F e 2 + 0.000 0.059 0.015 0.012 0.705 0.013 Note: The following standards were used: zircon (SiATa, ZrZa), spinel (ALfiTa), T h 0 2 (ThMx), diopside (CaKa), U 0 2 (UMa), H f 0 2 (HfLa), fayalite (FeA'a). Compositions were recalculated on the basis of 16 O apfu and maximum values are shown in bold. R E E s and Pb were sought but not detected. * indicates sample with highest total. 40 3.500 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 Fe2t (apfu) 0.025 0.005 0.000 <5&» 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 Fe2*{apfu) Figure 18. Iron, Si, Zr and Al content of zircon as determined via EMPA. (a) shows a correlation between Zr and Si loss with Fe gain, (b) shows a rough correlation between amount of Fe and Al. 41 Possible destroyed primary minerals Mortensen (1979) observed sodic amphibole (riebeckite) in the felsic metavolcanic rocks and Chronic (1979) noted potassic amphibole and pyroxene (arfvedsonite and aegirine) in less-altered syenite samples. However, petrographic examination of samples collected from within the study area shows that all primary ferromagnesian minerals have been completely destroyed through alteration. A literature review of syenites emplaced in extensional intra-plate environments was undertaken for comparison of mineral assemblages. Common major phases described by Barker (1977), Nappi et al. (1998), White and Urbanczyk (2001), and Mushkin et al. (2003) include feldspar, amphibole, pyroxene, and less commonly quartz. Primary accessory minerals can be more varied, since alkali and trace element geochemistry are specific to each locality and w i l l have a large impact on resulting mineralogy. In general, common accessory minerals include zircon, phosphate minerals {e.g., monazite and apatite), fluorite, biotite, iron sulfide minerals, ilmenite, and magnetite. Secondary minerals Alkali feldspars Albite is observed in the syenite as an alteration product from Na-metasomatism acting upon K-feldspar. In thin section, the albite exhibits flame (replacement) perthite textures, evidenced by fronts of albite infiltrating by means o f alkali exchange of K for Na . This alteration typically occurs as rinds around K-feldspar and commonly completely replaces the original phenocrysts (Figure 16). Many of the albite flames show kinking (Figures 19), suggesting that alteration occurred either during or prior to deformation or Na-alteration progressed along previously deformed planes. Albite is also present in xenoliths within the syenite as radiating fan-shaped masses with smaller grain sizes. These fans contain dustings of Fe ± T i oxides (dominantly magnetite) along their centres. Electron microprobe data (Table 5) show that the replacement albite has an average composition of AbggOro.sAno.s-42 Figures 19. Photomicrographs (XPL) of deformed and dislocated flame perthite (albite) in syenite at the True Blue showing. 43 Phyllosilicates These include biotite, chlorite, and rare white mica. Biotite and chlorite occur both as masses with roughly polygonal outlines and along fractures and grain boundaries of feldspar crystals. Biotite tends to occur as coarser grains up to 0.25 mm and triple junctions between books have been observed (Figure 20). The chlorite is apple green in colour and occurs as finer-grained felted masses (Figure 21). The phyllosilicates do,not show any obvious preferred orientation. Biotite and chlorite are always spatially associated with iron oxides, carbonates, rarely with fluorite, and are often seen in proximity to flame perthite. White micas are seen along the perimeter of, and in fractures cutting feldspars. Compositions of phyllosilicates are given in Table 7. The general formula of biotite can be expressed as AR^O^Xji. Figure 22 depicts the. relationships between annite, phlogopite, siderophyllite, and eastonite members. In these trioctahedral micas the A site is usually populated by K and the R site is typically populated by F e 2 + or M g . The 7 site is occupied by A l and Si in a 1:3 or 2:2 ratio, and the X site is occupied by OFT, F", and Cl" . A l l but one sample plot within the annite quadrant. X M g ( M g / M g + Fe)apfu values range from 0.159 to 0.189 and average 0.171 while *A1 values range from 0.112 to 0.597 and average 0.188. F values up to 0.75 wt.% were detected and a rough correlation is seen between F a n d X M g (Figure 23). With the exception of one sample, all analyses show F > C l . Three samples contain anomalous values of Na , resulting in over-population at the A site. This appears to be correlated with r A l (Figure 24). These samples are also the outliers in Figure 22 that reside closer to the siderophyllite end-member. Figures 25 and 26 show that charge balancing in the i?-site and dominant exchange mechanisms are being achieved by the following: *T i + 2*A1 = 2RD AU + r S i = L v , N a + ^ K ] + 7 A l 44 Figure 20. Photomicrographs in P P L (a) and X P L (b) of secondary biotite in syenite from the True Blue showing. Biotite is most common along K-feldspar grain boundaries and fractures. Figure 21 .Photomicrographs in P P L (a) and X P L (b) of secondary chlorite in syenite from the True Blue showing. Chlorite is most common as clots with associated Fe-oxides, quartz, carbonate and fluorite. The one depicted here was subsequently cut by a quartz vein. 45 T A B L E 7. S E L E C T E D E L E C T R O N MICROPROBE COMPOSITIONS OF PHYLLOSILICATES F R O M T H E T R U E B L U E SHOWING ' Sample THIII04E THIII04E THIII04E THIII04E THIII04E DTTH01 DTTH01 DTTH01 -3-10 -5-30 -2-1 -2-5 -2-2 -1-15 -2-3 -1-12 Mineral annite annite annite annite ' annite chamosite chamosite chamosite S i0 2 (wt.%) 44.85 34.94 34.82 35.08 36.48 26.54 23.37 23.50 T i 0 2 1.90 2.68 2.34 2.36 2.09 0.02 0.23 0.06 A1 2 0 3 15.48 13.05 13.33 13.31 13.39 18.46 19.27 19.68 C r 2 0 3 0.00 0.00 0.00 0.05 0.05 0.00 0.06 0.02 MgO 2.60 3.37 3.52 3.51 3.85 6.06 6.18 6.34 CaO 0.04 0.00 0.14 0.03 0.01 0.05 0.01 0.04 MnO 0.01 0.14 0.95 0.13 0.10 0.08 0.10 0.10 FeO 22.79 30.78 30.08 32.57 29.46 34.46 38.36 38.79 BaO 0.06 0.18 0:16 0.12 0.19 0.01 0.06 0.02 Na 2 0 4.21 0.09 0.10 0.07 0.60 0.89 0.02 0.02 K 2 0 6.23 9.04 9.01 9.09 9,13 0.02 0.00 0.00 F 0.34 0.56 0.53 0.55 0.75 0.00 0.00 0.00 Cl 0.36 0.31 0.43 0.36 0.31 0.14 0.05 0.06 H 2 0 * 3.86 3.30 3.29 3.35 3.29 10.64 10.49 • 10.60 0=F -0.14 -0.24 -0.22 -0.23 -0.32 0.00 0.00 0.00 0=C1 -0.08 -0.07 -0.10 -0.08 -0.07 -0.03 -0.01 -0.01 Total 102.51 98.14 98.38 100.27 99.32 97.34 98.19 99.22 S i 4 + (apfit) • 3.268 2.873 2.859 2.841 2.934 2.982 2.668 2.655 T i 4 + 0.104 0.166 0.145 0.144 0.127 0.002 0.020 0.005 A l 3 + 1.329 1.265 1.290 1.271 1.269 2.445 2.592 2.620 C r 3 + 0.000 0.000 0.000 0.003 0.003 0.000 0.006 0.002 M g 2 + 0.283 0.413 0.431 0.424 0.462 1.015 1.052 1.068 C a 2 + 0.003 0.000 0.013 0.003 0.001 0.006 0.001 0.005 M n 2 + 0.001 0.010 0.066 0.009 0.007 0.008 0.010 0.010 Fe 2 + 1.389 2.117 2.065 2.206 1.981 3.238 3.662 3.664 B a 2 + 0.002 0.006 0.005 0.004 0.006 0.001 0.003 0.001 N a + 0.595 0.015 0.016 0.011 0.094 0.194 0.005 0.005 K + 0.579 0.948 0.944 0.939 0.937 0.003 0.000 0.000 F" 0.079 0.146 0.138 0.141 0.191 0.000 0.000 0.000 Cl" 0.045 0.043 0.060 0.050 0.042 0.027 0.010 0.012 H + 1.877 1.811 1.803 1.810 1.767 7.974 7.991 7.989 o2- 11.877 11.811 11.803 11.810 11.767 17.974 17.991 17.989 Cation Sum 7.551 7.811 7.832 7.854 7.819 9.893 10.016 10.032 Anion Sum . 12.000 12.000 12.000 12.000 12.000 18.000 18.000 18.000 Note: The following standards were used: fluorphlogopite (SiKa, MgKa, YJCa, FKa), rutile (TiKa), kyanite (AlKa), chromite (CrAxi), diopside (CaKa), rhodonite (MnKct), fayalite (Fe^a), barite (BaLa), albite (NaArx), and scapolite (ClArx). Compositions were recalculated on the basis of 2 (OH, F, Cl) and 12 O for annite and 8 (OH, F, Cl) and 18 O for chlorite. Maximum values are shown in bold. *Determined by stoichiometry. 46 1.000 0.800 o. 'r 0.600 0.400 0.200 0.000 siderophyllite o eastonite s 1 annite phlogopite 0.000 0.200 0.400 0.600 X ( M g )(apfu) 0.800 1.000 Figure 22. Classification of biotite from syenite. See text for discussion. 0 195 n 0 190 -0 185 -(apfu] 0 180 -a z 0 175 -x~ 0 170 -0 165 -0 160 -0 155 -0.000 0.050 0.100 0.150 F (apfu) 0.200 0.250 Figure 23. Biotite compositions from syenite. See text for discussion. o.ooo 0.000 0.200 0.400 0.600 'Na^apfu) Figure 24. Biotite compositions from syenite. See text for discussion. 0.800 47 1.400 1.200 1.000 5 0.800 ] Q. CN P 0.600 0.400 0.200 0.000 y= 1.0967x- 0.1961 R 2 = 0.9755 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 R T i4 + +>2*(KAI 3 +) (apfu) Figure 25. Charge balancing mechanism in biotite from syenite at the True Blue showing. 2.150 5 2.100 Q. CB ^ 2.050 H + CO z < + + 2.000 H 1.950 1.900 1.850 y = -0.995x + 4.9786 R = 0.997 2.800 2.850 2.900 2.950 3.000 3.050 3.100 3.150 A D + T S i 4 + (apfu) Figure 26. Substitution mechanisms in biotite from syenite at the True Blue showing. 48 Chlorite Xug (Mg/Mg + Fe)apju values range from 0.217 to 0.242 and average 0.232, placing these chlorite in the Fe-dominant chamosite (a.k.a daphnite) range. All chamosites analyzed are aluminous (AlT otai > 2 apfu, 7 7 Al > 1) with Al substituting at the M2 and MS sites up to 0;427. The chamosite is slightly Si deficient (<3 apfu) and M2 + MS site vacancy was noted in two samples. Trace CI was detected, however no F was found. Only minor amounts of Na, K, Mn, Ca, and Ba were detected, with the exception of one anomalous sample that contained 0.194 apfu Na. Carbonate minerals Secondary carbonate compositions vary greatly but include Ca, Fe, and Mg dominant species with minor Sr and Mn. The REE fluorocarbonate bastnaesite is also present. Carbonates occur along grain boundaries and in fractures of feldspars and are often associated with other alteration minerals such as oxides and phyllosilicates (Figure 27). Selected compositions of carbonate minerals in the syenite are listed in Table 8. Bastnaesite is Ce-dominant and also carries trace amounts of Th and U, as detected by EDS via the SEM. Bastnaesite recovered from a heavy mineral concentrate obtained through crushing of whole-rocks is orange-brown in colour and transparent to translucent and abundant material allowed phase identity confirmation through powder X-ray diffraction. EMPA analysis was attempted on bastnaesite but results did not allow formula recalculation. Large beam size, long count times, interference lines and decrepititation of the samples were problems encountered. However, individual REE oxides (e.g., La203) are consistent between samples as variations reach up to only ~3 wt.% (in La20a). All samples show enrichment in the LREE (Ce > La » Gd) and Ce203 and Gd203 reached up to 36.42 and 3.94 wt.%, respectively. Hsu (1992) investigated the stability of REE fluorocarbonates and determined that they are stable under magmatic and hydrothermal conditions (up to -750 °C). Wyllie et al. (1996) also note that REE-fluorocarbonate minerals could also form in lower temperature environments like carbonatites. The presence of bastnaesite and absence of synchisite ([REE,Ca,Th][C03]2F) is an indication of a low Ca system. Bastnaesite may have precipitated at the expense of monazite during a carbonation event, as was suggested by Johan and Johan (2005) in a granite cupola. 49 Figure 27. Photomicrographs (XPL ) of secondary carbonate in syenite at the True Blue showing. In (a) Ca-dominant carbonate is assoc ia ted with biotite along p lanes of weakness in f lame perthite (albite). In (b) bastnaesite and other REE-m ine ra l s are seen in proximity to f lame perthite 50 T A B L E 8. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S OF C A R B O N A T E M I N E R A L S F R O M T H E T R U E B L U E S H O W I N G Sample D T T H 0 2 D T T H 0 2 T H V 0 1 D T T H 0 2 T H V 0 1 1.-54 2-31 2-51 1-32 1-35 Location interstitial interstitial interstitial interstitial vein Mineral ferroan ferroan ahkerite ferroan ankerite dolomite dolomite dolomite M g O (wt.%) 17.86 17.05 6.64 16.53 8.64 CaO 30.51 30.52 28.01 30.11 28.17 M n O 0.18 0.24 0.87 0.31 0.85 FeO 5.42 6.69 22.16 6.74 19.67 SrO 0.14 0.07 0.10 0.16 0.24 co 2* 46.94 46.85 43.39 46.07 44.22 Total 101.05 101.42 101.17 99.92 101.79 M g 2 + (apfu) 0.831 0.795 0.334 0.784 0.427 C a 2 + 1.020 1.023 1.013 1.026 1.000 M n 2 + 0.005 0.006 0.025 0.008 0.024 F e 2 + 0.141 0.175 0.626 0.179 0.545 S r 2 + 0.003 0.001 0.002 0.003 0.005 Note: The following standards were used: dolomite (MgATa), calcite (CaXa), rhodochrosite (MnATa), siderite (FoKa), and strontianite (SriCa). Compositions were recalculated on the basis of 6 O and 2 C apfu. Maximum values are shown in bold. *Determined by stoichiometry. 51 Phosphate minerals Apatite, monazite and xenotime occur in areas of more intense alteration as grains up to approximately 200 urn in diameter. In thin section the phosphate minerals show second-order green and purple birefringence and have subhedral to anhedral morphology (Figure 28). They are commonly associated with carbonate and are typically near Na -altered feldspars and zircon. Oxide minerals Magnetite, ilmenite, rutile, and niobian rutile (ilmenorutile) were all observed in the syenite and selected compositions are given in Table 9. In thin section the iron-rich oxides typically have limonitic alteration rinds. Euhedral to subhedral magnetite and ilmenite crystals are commonly associated with phyllosilicates and albite. Anatase was found in a heavy mineral concentrate of a crushed rock sample as light blue fragments on the order of 1 mm. It was positively identified using single crystal X-ray diffraction. Anatase is typically considered to be the low temperature T i02 phase (Banfield et al. 1993, Zhang and Banfield 1999) and has been noted in association with greenschist facies zircon-altering synmetamorphic fluids (Lanzirotti and Hanson 1995) and ankerite-forming carbonating fluids (Seckendorff et al. 2000). Quartz Quartz is a minor phase that is spatially associated with secondary phyllosilicates. It is subhedral to anhedral with small grain sizes and is often locally enclosed by carbonate. Other accessory minerals Pyrite is observed in the syenite as small euhedral to subhedral grains and is likely of secondary origin (Figure 29). Fluorite is typically light to dark purple with grain diameters less than 1 mm. Some of these fluorite crystals may represent late primary minerals formed within interstices of the syenite although it is predominantly observed in areas of intense alteration. Chronic (1979) noted trace amounts of secondary epidote within the syenite. 52 Figure 28. Photomicrographs in PPL (a) and XPL (b) of secondary monazite in association with albite, carbonate and Fe-Ti-Nb oxides in syenite. Figure 29. Photomicrograph in reflected light (a) and BSE image (d) of subhedral to euhedral pyrite in syenite at the True Blue showing. The loose pyrite grain in (b) was obtained through conventional crushing of rock sample THV01. 53 T A B L E 9. S E L E C T E D E L E C T R O N M I C R O P R O B E C O M P O S I T I O N S OF O X I D E M I N E R A L S F R O M T H E T R U E B L U E S H O W I N G Sample T H V 01-53 D T T H 02-54 T H V 01-48 T H V 01-50 T H V 01-52 Location host vein host vein vein Mineral ferro-columbite niobian rutile rutile ilmenite ilmenite N b 2 0 5 (wt%) 68.95 10.43 0.47 0.03 0.11 T a 2 0 5 2.58 0.26 0.00 0.05 o.oi ; S i 0 2 0.01 0.09 0.16 0.01 0.00 T i 0 2 5.78 80.37 99.08 51.47 52.15 C r 2 0 3 0.00 0.00 0.00 0.01 0.00 M g O 0.16 0.00 0.00 0.04 0.05 CaO 0.03 0.42 0.05 0.00 0.01 M n O 0.42 0.00 0.04 0.38 0.34 FeO 20.10 5.18 0.40 48.31 48.01 Total 98.03 96.75 100.20 100.30 100.68 N b 5 + (apfu) 1.768 0.206 0.008 0.000 0.001 T a 5 + 0.040 0.004 0.000 0.000 0.000 S i 4 + 0.000 0.004 0.006 0.000 0.00 T i 4 + 0.246 2.632 2.974 0.982 0.988 C r 3 + 0.000 0.000 0.000 0.000 0.000 M g 2 + • '0.014 0.000 0.000 0.002 0.002 C a 2 + 0.002 0.020 0.002 0.000 0.000 M n 2 + 0.020 0.000 0.002 0.008 0.007 F e 2 + 0.952 0.188 0.014 1.024 1.011 o 2 - 6.000 6.000 6.000 3.000 3.000 Note: The following standards were used: columbite (NbZa), microlite (TaMa), diopside (SiiTa, CaiTa), rutile (T\Ka), chromite (Mg^a, M a ) , rhodonite (MnKa), and fayalite (Fe^Ta). Compositions were recalculated on the basis of 6 or 3 O apfu. Maximum values are shown in bold. A l , V , and N i were sought but not detected. 54 Interpretation of syenite mineralogy Introduction Primary minerals provide information pertaining to the geochemistry and conditions in which the syenite was formed. Secondary minerals provide information pertaining to the events which affected the syenite, but can also provide information about the original character of the rock. A t the True Blue occurrence, the majority of the primary assemblage has been destroyed and the rock dominantly comprises secondary minerals. Discussion of alteration observed in the Pelly Mountain Volcanic Belt as well as the origin of flame perthite can provide insight to the sequence of events that resulted in the observed assemblage. Furthermore, the explanation of the observed assemblages also has implications for the origin of the fluid that formed the mineralized tension gashes. V M S assemblage Studies of the P M V B by Mortensen (1982), Mortensen and Godwin (1982), and Hunt (1997, 2000) and mineral exploration reviews by Doherty (1997) and Gibson et al. (1999) indicate that significant alteration occurred during the formation of the V M S occurrences. Mortensen (1982) showed evidence of significant alkali exchange between N a and K in the felsic metavolcanics, as well as the development of clay and carbonate alteration. Hunt (1997, 2000) also states that the metavolcanic package has undergone pervasive clay and carbonate alteration and Doherty (1997) notes that extent of alteration is variable, but consists mainly of ferroan carbonate-quartz-sericite-pyrite assemblages with occasional fluorite and tremolite-actinolite. From these observations, it can be summarized that the metavolcanic rocks were subsequently enriched in Si , Na , CO2, and Fe, and depleted in K . This type of assemblage may have also been imprinted on the syenite since hydrothermal convection cells are commonly developed adjacent to high-level intrusions associated with V M S deposits (Hart et al. 2004). 55 Flame perthite A variety of perfhitic textures are present in feldspar minerals (e.g., film, braid, string, plate, etc.). They are most commonly ascribed to exsolving phenomena of K-rich feldspar from Na-rich feldspar during cooling of a magmatic system. Perthites can also be of secondary origin, typically owing to the alteration of K-feldspar by albite. Flame perthite is postulated to be of secondary origin and is most common in granitoids deformed under greenschist to amphibolite facies conditions (e.g., Pryer and Robin 1995, Lauri and Manttari 2002, and Vernon et al. 2004). Pryer and Robin (1995) reviewed the development of flame perthite during retrograde metamorphism and offer an explanation for its presence that involves several local reactions. They describe cyclic reactions where no external fluids are necessary to develop the observed assemblages, and that the extent of alteration and temperature at which it occurs is controlled by the amount of initial fluid (more fluid = more alteration and cyclic reactions induced at lower temperatures). Vernon (1999) also describes similar reactions in metapelitic gneisses. Both studies suggest that plagioclase (± cordierite) alters to muscovite and epidote, releasing Na used in alkali exchange during albite flame development in K-feldspar. This generates the release of K, which induces the replacement of more plagioclase by muscovite and therefore the release of additional Na. Si and A l are considered to be immobile. Pryer and Robin (1995) gave the following reaction: 20 oligoclase + 1 K-feldspar + 2 H 2 0 = 2 zoisite + muscovite + 2 quartz +15 albitepiagiociase + 1 albiteflame A similar setting, but with distinct differences, is seen at True Blue. Flame perthite is common and only rare K-feldspar cores remain. Additionally the primary geochemical signature of the granitoid at True Blue is different than rocks previously investigated. In the publication by Pryer and Robin (1995) the granitoids studied were not fully described, but were noted to contain assemblages of dominantly oligoclase and K -feldspar. This differs from the True Blue syenite, in that the original igneous composition was likely K-feldspar dominant, the syenite may have undergone syn-emplacement 56 alteration similar to the P M V B (addition of S i , Na, C O 2 , and Fe and depletion of K ) , and whole-rock geochemistry indicates low CaO concentrations. In contrast to the system described by Pryer and Robin (1995), the N a involved in cyclic reactions at True Blue could be sourced from alkali amphibole and pyroxene minerals (e.g., riebeckite, arfvedsonite, and aegirine) or syn-emplacement Na-alteration products, and the sink for K could be annite. Breakdown of alkali-amphibole and pyroxene minerals would also release Fe and possibly trace elements such as T i and R E E , which could then be sequestered by the abundant Fe±Ti±Nb oxide minerals and Fe±REE carbonates. In summary, the presence of flame perthite in the syntectonic alteration assemblage at the True Blue showing supports greenschist facies conditions. It also suggests that i f abundant N a were present prior to regional metamorphism, no external fluid would be required to produce the flame perthite. Brief timeline of reactions / interpretation A brief summary of events prior to veining at True Blue and pertinent to the syenite is warranted at this time. A K-feldspar dominant syenite was emplaced into an extensional environment within the Cassiar Platform during the Mississippian. The high-level intrusion gave rise to felsic volcanics and also assimilated country rock that included platform sediments and felsic volcanics. During this time hydrothermal convection cells were likely generated proximal to the syenite and the resulting alteration that affected the P M V B (addition of Si , Na , C O 2 , and Fe and depletion of K ) may have also affected the syenite. Late-stage magmatic autometasomatic alteration may have also occurred, as has been reported for granite and syenite intrusions (e.g., Johan and Johan 2005). These fluids often have high N a and rare-element concentrations. East-north-east directed compressional tectonics and associated regional metamorphism began during post mid-Triassic, regional metamorphic grades reached greenschist facies, and a general assemblage of albitefia me ± annite ± chamosite ± magnetite ± Fe-REE-carbonate ± monazite was developed. 57 C H A P T E R 5 ' r M I N E R A L O G Y A N D G E O C H E M I S T R Y O F T H E B E R Y L - B E A R I N G V E I N S Overview of morphologies, textures and vein origins Veins have been studied in detail by many authors using a variety of approaches resulting in a variety of interpretations. Recent reviews by Oliver (1996), Bons (2000), and Oliver and Bons (2001) were used as guides for terminology and interpretation of observations. V e i n textures are important when determining the history of veins, including the source of fluid and mechanisms for precipitation. Accordingly, objectively describing veins and their host rock is important since the interpretation of particular textures can change with additional research. Veins can be described via three categories (Bons 2000): Macroscopic morphology, microscopic morphology, and growth morphology. Additional geochemical and mineralogical information can also be tagged on after these categories have been addressed. Macroscopic morphology refers to general shape and includes terms such as tension, shear, breccia, and pressure fringe. Microscopic morphology refers to the arrangement and shape of minerals within a vein and includes terms such as blocky, elongate blocky, fibrous, and stretched. Growth morphology refers to where crystals nucleate and how they grow, and includes terms such as syntaxial, unitaxial, antitaxial, and composite. The terms mentioned here are small samplings of those encountered in the.literature and many terms belong only to specific vein systems. Chemical and mineralogical modifiers can be added to a vein description and can enhance the information presented, especially i f the constituents are unusual or have particular significance. Two main groupings of veins are "open" and "closed" (Oliver 1996, Bons 2000, Oliver and Bons 2001). A n "open" system is one which derives the majority of the vein mass from an external source. A "closed" system is one which derives the majority of the vein mass from a local source within the defined system. Other variables can be considered when defining a system, such as the presence of fractures, which then results in subgroups, such as "closed system-fractured". The number of approaches in which one 58 can subdivide is extensive and includes variables such as wallrock permeability, wallrock heterogeneity, mode of fluid transport, wallrock-fluid equilibration, geometry of fluid flow, and amount of fluid flux, among others. The interaction between a fluid and its host rock perhaps reveals the most information pertaining to conditions at the time of vein formation. In general, the collection of observations assembles a set of conditions within which a system has operated. Interpretation of these conditions then allows a history of vein formation to be developed. Macroscopic morphology of veins at the True Blue occurrence Beryl (var. aquamarine) at the True Blue showing occurs in quartz-filled tension gashes within a rare-element-enriched syenite stock. These sigmoidal tension gashes (Figure 11) are most common within the syenite near the contact with the metavolcanic wall rocks at higher elevations. A t lower elevations and towards the 'core' of the syenite, the tension gashes decrease in both size and abundance. Rare tension gashes emplaced into the metavolcanic rocks have been documented to contain fluorite, siderite, minor sulfides, and quartz, but neither beryl nor allanite. A t an outcrop scale, the tension gashes appear to be isolated from one another; however, investigation of thin sections revealed sub-mm sized veins that suggest possible communication between veins. Contacts with the host syenite are typically sharp and distinct vein selvages are not observed at outcrop or thin section scales. Intensity of radioactivity did not help to differentiate between beryl-bearing and barren veins, or between veins with lighter or darker coloured beryl, as determined by a field scintillometer survey. Minera logy i n order of abundance Quar tz Quartz exhibits blocky and elongate blocky textures and commonly has undulatory zoning. It is rarely seen as inclusions in beryl and carbonate and is most often the last phase to crystallize, filling the width of the veins. Inclusion trails, bands of wall rock, and fluid inclusions are all common in the quartz crystals (Figure 30). 59 Figure 30. Photomicrographs in XPL (a) and PPL (b) of quartz veins cutting syenite, (a) shows elongate blocky textures while (b) shows bands of wall rock inclusions, suggesting multiple crack-seal events. Dashed line in (a) and (b) follow the sharp wall rock contacts. 60 Carbonate minerals There are two generations of carbonate minerals but not all veins contain both. Siderite and ankerite crystallized early, nucleated from vein walls, and are the dominant species of carbonate. Selected E M P A analyses are given in Table 8. Ferroan dolomite is less common, appears to have co precipitated with quartz, and occurs towards the centers of the veins. The carbonate minerals are typically euhedral and range in size from microscopic to several em's in width. Siderite and ankerite are commonly altered with rims and fractures of limonitic material (Figure 31). Figure 31 . Photomicrograph ( P P L ) and B S E image inset of siderite bearing quartz-fi l led tension gash cutting biotite altered syenite. 61 Beryl Beryl crystals range in colour from light blue to dark blue, and from yellow-green to green to turquoise. The dark blue beryl found at the True Blue showing is among the darkest natural blue beryl in the world, although documentation of the other sources is limited. Localities mentioning dark blue beryl include Lone Pine (United States, Murdoch and Webb 1966), Ambositra (Madagascar, Pezzotta 2001), Gilgit (Pakistan, Kazmi et al. 1985), V a l Strem (Switzerland, Falster 2002) and a small number of localities in Brazi l (e.g., Marambaia and Paraiba, Proctor 1984). Beryl is most commonly observed as euhedral crystals and single crystal sizes discovered thus far range up to -2.5 cm in width and ~5 cm in length. Clusters of crystals have been found to measure up to 8 by' 9 cm. The beryl crystals are typically translucent but range from opaque to transparent. Fractures perpendicular to the c axis are the primary cause for degradation of clarity and solid inclusions are the next most significant cause. Euhedral to anhedral cores (single and multiple) and complex internal zoning are present in many crystals when observed in P P L but are more distinct in B S E images (Figure 32). Mineral inclusions include Fe-bearing carbonate, quartz, and a calcium-REE phase. Pseudomorphs and alteration minerals replacing beryl comprise pyrophyllite, Fe-bearing carbonate, green micas, and opaques. A total of 192 analyses of major and minor elements from more than 25 beryl crystals of varying colour were acquired by E M P A and are summarized in Table 10. Significant amounts of Fe, Na , Ca, and M g were detected. O f these cations, FeO values average 3 wt.% and range up to 5.92 wt.%, the highest values ever reported for true beryl. The only member of the beryl family with more Fe is stoppaniite, which contains up to 19.30 wt.% F e 2 0 3 (Delia Ventura et al. 2000). Figure 33a shows typical substitution at the octahedral (Al) site by Fe, M g , Sc, V , Cr, and M n . The trend indicated by the data is offset above the 1:1 line by approximately 0.05 apfu. Figure 33b shows divalent octahedrally coordinated cations versus monovalent cations residing at the channel site. Deviations from the 1:1 trends suggest that some of the Fe is in fact ferric, some of the Fe may reside in the channel site, and/or some of the Fe may reside in an interstitial site. Samples 21 and GS2 are dark blue and appear to have distinct trends as seen on Figures 33a and 33b. 62 Figure 32. BSE images (a, b, d) and PPL photomicrographs (c) of multiple anhedral to euhedral cores and complex internal zoning within beryl. In BSE images, lighter tones indicate higher average atomic mass and are likely correlated here with increased iron content. TABLE 10. SELECTED ELECTRON MICROPROBE COMPOSITIONS OF BERYL FROM THE TRUE BLUE SHOWING Sample GS2 21B 38 56 EXT46 TH07 BW4 UNC GBZ THV01 Suite Suite 1-2^ -4 -11 -04 -21 -3 -3 -7 -5 -10 min max Colour//7 Dark blue Dark blue Medium blue Medium blue Medium blue Light blue Light blue Light blue Dark green Light green/ yellow 192 192 Si0 2 (wt.%) 62.05 62.98 62.39 62.29 62.64 62.86 62.76 63.06 63.69 64.08 61.42 64.31 Ti0 2 0.03 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 A l 2 0 3 11.49 12.07 13.19 13.53 12.62 13.25 13.71 12.84 14.32 14.97 11.49 15.10 SC2O3 0.03 0.00 0.00 0.03 0.04 0.00 0.03 0.00 0.00 0.00 0.00 0.07 V2O3 0.00 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 C r 2 0 3 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 BeO* 12.88 13.02 12.99 12.99 13.03 13.11 13.07 13.11 13.28 13.34 12.74 13.40 MgO 1.89 1.81 1.93 1.68 2.51 2.58 1.77 3.26 1.55 1.58 1.29 3.42 CaO 0.00 0.00 0.01 0.00 0.00 0.09 0.05 0.03 0.00 0.00 0.00 0.11 MnO 0.00 0.00 0.00 0.03 0.00 0.07 0.00 0.00 0.00 0.00 0.00 0.10 FeO 5.92 5.51 3.33 3.29 3.39 2.39 2.84 1.54 3.03 1.83 1.47 5.92 Na 2 0 2.45 1.87 2.23 2.24 2.39 2.44 2.20 2.54 1.99 1.82 1.70 2.66 K20 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 C s 2 0 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.13 H 2 O t 2.92 2.43 2.73 2.74 2.87 2.91 2.71 2.00 2.53 2.38 2.28 3.10 Total 99.72 99.69 98.86 98.87 99.49 99.70 99.14 99.38 100.39 100.00 97.90 100.79 S i 4 + (apfu) 6.017 6.040 6.000 5.988 6.002 5.986 5.996 6.007 5.991 « 5.997 5.975 6.043 T i 4 + 0.002 0.000 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.004 A l 3 + 1.313 1.364 1.495 1.533 1.425 1.487 1.544 1.442 1.587 1.651 1.313 1.661 S c 3 + 0.003 0.000 0.000 0.003 0.003 0.000 0.002 0.000 0.000 0.000 0.000 0.006 v 3 + 0.002 0.000 0.001 0.002 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 Cr 3 + 0.001 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.004 Be 2 + 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 Mg 2 + 0.273 0.259 0.277 0.241 0.359 0.366 0.252 0.463 0.217 0.220 0.179 0.483 C a 2 + 0.001 0.000 0.001 0.000 0.000 0.009 0.005 0.003 0.000 0.000 0.000 0.011 Mn 2 + 0.000 0.000 0.000 0.002 0.000 0.006 0.000 0.000 0.000 0.000 0.000 0.008 Fe 2 + 0.480 0.442 0.268 0.264 0.272 0.190 0.227 0.123 0.238 0.143 0.117 0.480 Na + 0.461 0.348 0.416 0.417 0.444 0.451 0.408 0.469 0.363 0.330 0.313 0.492 K+ 0.002 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.005 C s + 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.008 Note: The following standards were used: albite (SiKa, AlKa, NaKa), rutile (TiKa), Sc metal (ScKd), V metal (VKa), chromite (CrKa), diopside (MgKa, CaKa), rhodonite (MnKa), fayalite (FeKa), orthoclase (KKa), and pollucite (CsLa). Compositions were recalculated on the basis of 3 Be and 18 0 apfu. *Determined by stoichiometry. Calculated using H 2 0 = (0.84958 x Na 20) + 0.8373 (G. Giuliani, pers. commun.). 0.80 CD + O CO + > + O + + 0 LL 0.50 0.20 1.50 Al (apfu) 1.80 0.85 o + + CD 0.55 0.25 0.25 0.55 Fe + Mg {apfu) 0.85 Figure 33. (a) Al versus the sum of other 0-site cations. The line represents 1:1 substitution, (b) Fe + Mg versus the sum of monovalent cations (Na + K + Cs) at the channel site. The line represents charge balance between divalent O-site . cations and monovalent channel occupants. Samples have been divided in the legend by colour: GS2 and 21 are dark blue; GBZ is green; THV01 is yellow; 'Normal' samples are pale to medium blue to turquoise. 65 Visible-Near Infrared (Vis-NIR) light absorbance data collected parallel and perpendicular to the c axis from the GS2 sample shows that maximum absorbtion in the 500-1200 nm range is oriented parallel to the c axis. These patterns do not correlate with absorptions known to originate from octahedral occupancy and consequently suggest occupation of another site to explain this absorption. Valence states and coordination states of 5 7 F e in natural materials can often be deduced from Mossbauer spectroscopy. This spectroscopic technique utilizes gamma radiation absorption in conjunction with the Doppler effect to detect minute, or hyperfine, variations in the nuclear environment of 5 7 F e . Variables such as coordination number and valence state of the lattice-bound Fe atoms give rise to particular modifications o f the collected spectra. A description of Mossbauer spectroscopy applied to the earth sciences can be found in a number of publications (e.g., Mit ra 1992). Unfortunately beryl produces unusual Mossbauer spectra that are not always fully understood, thus making interpretation with this technique difficult. In fact, papers dealing with beryl Mossbauer spectra commonly use words such as 'intriguing' and 'unusual', or 'never before noted' (Viana et al. 2002, Viana et al. 2001). Mossbauer data collected at 14 K (Figure 34) by. Dr . D . Dyar from the sample GS2 has not been entirely conclusive for determining the relative amounts and positions of ferrous and ferric iron. However, the data has indicated at least five major unique states for iron in the sample. Two unique states can be ascribed to 2+ and 3+ °Fe in beryl (47% and 18%, respectively), while one appears to be the result of an Fe-bearing impurity (19%). The remaining two appear to be tetrahedrally coordinated with a 2+ valence state (7% and 9%); however it is difficult to ascribe a location for them. Additionally, there are several places in the data that are not well accounted for by the current modeling at 14 K . Collection of additional data on beryl at 4.2 K and siderite at 14 K by Dr. D . Dyar is planned; however, it is scheduled to happen in late December 2005 at the earliest. 66 ON 99.5 98.5 97.5 c ~ 96.5 a h. o to si < 95.5 94.5 93.5 92.5 -12 -10 -8 Modeled Spectra Subset 1 - 47% Subset 2 - 9% Subset 3 - 1 8 % Subset 4 - 1 9 % Subset 5 - 7% -2 0 2 Velocity (mm/s) 8 10 12 Figure 34. Mossbauer spectra of dark blue beryl sample GS2 . Data was collected at 14 K. Colour in beryl Colour-causing chromophores have a variety of effects on the colour of beryl, depending on location, concentration, and valence state of the particular element or set of elements. The most commonly known example of colour-inducing substitution is that of Cr , which imparts a vibrant green colour to the variety of beryl known as emerald. Iron is a special circumstance because it can enjoy both 2+ and 3+ valence states and enter the C, T2 and O sites and may also sit at typically unoccupied interstitial sites (for the purposes of this section, the interstitial site shall be referred to as / ) . Furthermore, colour produced by the presence of iron varies depending on the combination, or combinations, of site and valence state. Historically, interpretations of colour from iron have been varied. For instance, Goldman et al. (1978) and Fritsch and Rossman (1988) describe yellow, green, and blue hues as being determined by F e 3 + / F e 2 + ratios and the effects of inter-valence charge transfer ( IVCT) while Wood and Nassau (1968) ascribe the blue colour to F e 2 + at the C site and yellow to F e 3 + at the O site. Two recent studies by Viana et al. (2002) appear to confirm and expand on both opinions. In brief, their data suggest that colour is defined by the relative abundance of Fe (blue) and Fe (yellow) and that ° F e 2 + produces no colour. A s noted, colour in beryl from True Blue ranges from light blue to dark blue, and from yellow-green to green to turquoise (Figure 35). Colour zonation has also been noted in several crystals and is characterized by blue cores surrounded by green rims. These crystals typically occur adjacent to allanite (Figure 35c), which suggests the chromophore, likely iron, is partitioning favourably into that phase. However, in addition to chromophore concentration, valence state, and location of the chromophore are also important factors for determining colour. Clusters of beryl crystals that show variations in colour between the crystals commonly have Fe-bearing phases, such as allanite or ilmenite, in close proximity to the lighter blue or greener beryl crystals. Dark-blue beryl crystals in contact with earlier siderite do not show colour zoning (Figure 35a). 68 Figure 35. Photographs of beryl and associated minerals within quartz veins, (a) and (b) show beryl precipitating before and after siderite. (c) and (d) show beryl co-precipitating with allanite; note the colour zoned beryl in (c). (e) and (f) are examples of euhedral gem quality dark blue beryl, (g) and (h) exhibit green colouration and differences in aspect ratios. The 'cream' coloured mineral in (g) is fluorite. All scale bars in photos are 1 cm spaces with 1 mm increments. 69 Referring to the latest data by Viana et al. (2002), the cause of blue in the True Blue beryl may be attributed to F e 2 + situated elsewhere than at the octahedral site. Unfortunately, the valence state of Fe cannot be determined directly via E M P A , and as noted earlier Fe can assume both 2+ and 3+ valence states. This makes it difficult to define the position and valence of iron. However, by attempting several approaches and considering their assumptions, some fairly concrete statements can be made about the crystal chemistry of beryl, and therefore potential causes of colour. It is pertinent to note here that Be is not detectable via conventional E M P A , and therefore has been assumed to have perfect occupancy (no substitutions) in these beryl samples. Additionally, all Fe was assumed to have a 2+ valence state during E M P A analysis. Most refinements and site designations for beryl consider octahedral substitution as the most common way to deal with deficient A l and excess 2+ and 3+ cations. Figure 33 showed this approach for the True Blue samples. For those samples with low substitutions and low channel constituents, this approach appears sufficient. However, the deviations from perfect correlations in the samples with greater degrees of substitution are cause for reinvestigation. Charge balancing is required for proper site assignment in any mineral. If only octahedral, channel hosted, and interstitial substitutions are considered in the beryl analyses, then undercharging at the O site should be accommodated by interstitial (7) and channel (C) overcharging. I f we allow enough ° F e 2 + to migrate to the 7 or C site to balance the charge, Figures 36a and 36b are obtained. Figure 36a shows perfect charge balancing and Figure 36b shows acceptable occupation of the O site. However, this approach does not consider the presence of possible F e 3 + . If charge balancing between the O and I+C sites is achieved only through the assignment of Fe to Fe , Figures 37a and 37b are produced. These indicate that balancing can be achieved for most samples, however surplus occupation of the O site is still a problem. This approach neither allows Fe to migrate into the I or C sites, nor does it take into consideration charge balancing at the 77 or 72 sites. 70 -0.700 -0.650 -0.600 -0.550 -0.500 -0.450 -0.400 -0.350 -0.300 overcharge 0.3 0.4 0.5 0.6 Charge Surplus at Channel Site (cFe2* + cNa ,* + cK ,* + cCs1*) 0.8 £ 0 . 7 (0 + c . 0"i_ .Q <~ 3 + <f>i 0.6 - 0.5 C S + + ~®0.3 0.2 A GS2 o 21 • GBZ x THV01 + Normal 0.7 1.2- 1.3 1.4 1.5 O A | 3 t 1.6 1.7 1.8 Figure 36. Crystal chemistry of beryl. See text and Figure 33 for further detail. r N a ^ + ' t C + 'Cs1*) Figure 37. Crystal chemistry of beryl. See text and Figure 33 for further detail. 71 Figures 38a and b are the result of considering substitutions at the Tl site, but leaving ideal occupancy of Be. The Tl site was filled with Si and then any deficiencies were filled with A l , while surplus Si was moved to the O site. Following this, the O site was then filled with regular constituents, and then with enough F e 2 + to bring occupancy to the ideal value of 2. Surplus F e 2 + was considered to occupy either the I or C site. A s shown in Figure 38a, overcharging and undercharging at the various sites achieves charge balance within reason, and thus the occupancy of the I or C site by F e 2 + is supported. Figure 38b also supports this statement, as it indicates that monovalent cations cannot be the only counter effect for divalent substitution at the O site. A s a note, a graph with O* substitution and °Al was not presented, as a perfect correlation was arithmetically forced. The approach used to generate Figure 39a, b, and c is similar to the last approach, however the excess Si was assigned to the T2 site, and a corresponding amount of Be was deemed missing. F e 2 + assignment followed the same steps. Figure 39a shows that all analyses indicate an overall undercharge and Figure 39b, like Figure 38b, supports the presence of Fe at either the I or C site. Figure 39c shows that no distinct correlations exist between deficient Be (i.e., excess Si) and F e 2 + at the 7 or C site. Thus far this last approach has generated the most agreeable results. However, the undercharging of the samples can be quickly attended to by allowing some o f t h e ° F e 2 + to enjoy life as ° F e 3 + . Using the basis from the last approach but allowing °Fe to change valence state results in an arithmetically balanced system as shown in Figure 40a. Figure 40b, as with the others, again supports the existence of F e 2 + at either the I or C sites. Thus, from this last approach virtually all samples achieve perfect occupancy at the Tl, T2, and O sites and result in charge balancing through the presence of monovalent cations at the C site and divalent Fe at the C and/or I sites. 72 Overall Charge Surplus Sum of Divalent Octahedral Cations (°Si4* + °Ti4* + cNa1* + CK1* + cCs1*+ 2*c"Fe2*) Figure 38.- Crystal chemistry of beryl. See text for further detail. % + os o f ! 5 ° (0 o « + > s ° < undercharge overcharge -0.300J 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 Overall Charge Surplus (2*T2Si4* + 0 Ti4* + c Na1* + 0 K1* + c Cs1* + 2*c" Fe2' Figure 39. Crystal chemistry of beryl. See text for further detail. 1.000 -| m c 0.900 -o +3 re 0.800 -O nel 0.700 -c re - C 0.600 -U c 0.500 -O ro > 0.400 -o c o 0.300 -S o 0.200 -E Su 0.100 -0.000 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 Sum of Divalent Octahedral Cations . * $ * 0.000 2.950 2.955 2.960 2.965 2.970 T2f3e2* 2 9 8 5 2 , 9 9 0 2 9 9 5 3 0 0 0 3 0 0 5 73 74 Now that a satisfactory site assignment has been achieved for all samples, inferences can be made regarding the causes of colour in beryl. Goldman et al. (1978) and Rossman (1981) ascribe colour to F e 3 + / F e 2 + ratios and I V C T . Figure 41a shows that using ratios alone results in dark blue samples plotted alongside light blue and yellow 04- f~i ^4-samples. Wood and Nassau (1968) ascribe the blue colour to Fe and yellow to Fe . Figure 41b shows Fe plotted against Fe , but does not appear to fully explain the colour variations seen in the samples from True Blue. The hypothetical colour vectors are not absolute in this diagram, and i f the intensity of colour generated by c F e 2 + greatly exceeds that of Fe , the "yellow + blue" vector may have a much shallower slope. It is interesting to note that site occupancies calculated for the yellow samples results in little to no ° F e 3 + . The fact that neither of these approaches can satisfactorily differentiate dark blue samples from the rest of the population suggests that there is another mechanism that may be responsible for colour. The percentages of each site assignment for Fe are plotted in Figures 42a, b, and c. From these diagrams it can be said that only proportions of these variables w i l l not define the colour. Thus, to differentiate between samples, the amount of Fe must also play a role. Indeed, Figures 43a, b, and c w i l l all show some autocorrelation between axes because of the total amount of Fe in each sample, however statements can still be made regarding the cause of colour. If Figures 43 a and 43b are divided into approximate high and low.quadrants, only the dark blue samples are positioned in the "high / / c F e 2 + - high O 2"!" I/C 2+ Fe " and "high Fe - high Fe-rot" areas. In Figures 41b and 43c the dark blue samples are spread across the ° F e 3 + axis and juxtaposed against the less-saturated samples. Thus, what distinguishes the darkest samples from the remainder is the total amount of F e 2 + at both the I/C and O sites. However, some the studies mentioned previously have shown that ° F e 2 + does not impart colour to beryl. Therefore, the distinctly darker blue beryl from the True Blue showing is likely the result of minute amounts of F e 2 + at either the C or I site. 75 0.3 0.000 0.000 0.020 0.040 0.060 °Fe 3 + 0.080 0.100 Figure 41. Crystal chemistry of beryl. See text and Figure 33 for further detail. 76 CO 100 90 80 70 60 50 H 40 30 20 _iA. + O O i a GS2 o 21 • GBZ x THV01 + Normal 0 10 + 15 20 %°Fe 3 + 25 + 30 35 CD 50 45 4 0 , 35 30 25 -20 -15 -10 5 0 + + A GS2 o 21 ° GBZ x THV01 + Normal 20 40 + 60 % 0 Fe 2 + 80 50 45 40 CD + + 100 + a GS2 o 21 • GBZ x THV01 30 40 Figure 42. Crystal chemistry of beryl. See text and Figure 33 for further detail. 77 0.100 0.090 0.080 0.070 0.060 CM £ 0.050 y 0.040 0.030 0.020 0.010 0.000 o .0 A o o AGS2 o 21 • GBZ x THV01 + Normal 0.400 0.350 0.300 0.250 + "j® 0.200 o 0.150 0.100 0.050 0.000 A + + • A A A O A A4b A A A A /s r-rV+ + + 0.000 0.100 0.200 0.300 0.400 Total Fe 0.500 0.600 0.000 0.020 0.040 0.060 „C p e 2 + A GS2 o 21 o GBZ x THV01 + Normal 0.080 0.100 0.400 0.350 0.300 0.250 ,? 0.200 0.150 0.100 A 0.050 A A AA O A A . A A& A & 0.000 0.000 0.020 0.040 0.060 o o a GS2 o 21 • GBZ x THV01 + Normal 0.080 0.100 ° F e 3 Figure 43. Crystal chemistry of beryl. See text and Figure 33 for further detail. 78 A s noted previously, the V i s - N I R absorption spectra of the dark blue beryl suggest that the chromophore(s) is(are) sitting parallel to the c-axis. The I site best satisfies this condition and Mossbauer data confirms the presence of F e i n an unusual position. The large percentages of octahedrally coordinated F e 2 + and F e 3 + l ikely reside at the O site in place of A l . The remaining Fe in the two unusual states is likely the Fe that is causing the distinctly dark blue colour, however, Fe-bearing inclusions within the beryl may also be accountable. Investigating information obtained from single crystal X-ray and neutron studies performed by Dr. L . A . Groat using the darkest of the True Blue beryl (sample GS2), further statements can be made. In that data, no detectable Fe was found to exist in the channel sites. I f in those samples with the highest amount of F e 2 + in the I/C no detectable Fe was present in the channels, it is safe to state that the unusual colour saturation in the darker blue beryl is a result of F e 2 + sitting within an interstitial site (I). Although sensitivity to the low amount (average occupation of 0.050 apfu, up to 0.087) of Fe at the I site is low, the small amounts of chromophores needed to cause intense colour saturation supports the assumption that Fe is the most likely candidate for the markedly dark blue beryl from the True Blue showing. Interestingly, published results of other 'dark blue' and 'blue' beryl only ascribe values (i.e. apfu) to ° F e 2 + , do not appear to have any site deficiencies, and sometimes state the presence of c F e 2 + from other approaches but do not give any occupancy values (Aurisicchio et al. 1988, Viana et al. 2002a, Viana et al. 2002b, Taran and Rossman 2001, Ar t io l i et al. 1993, Ar t io l i et al. 1995, Isotani et al. 1989). Al lani te Allanite is part of the epidote group of minerals. What distinguishes allanite from others is its essential R E E 3 + and F e 2 + components, ideally 1 apfu each that reside at the A2 and M3 sites, respectively. A crystal chemistry paper by Ercit (2002) dealt with several approaches for normalizing E M P A data, while a more recent overview of R E E epidote is given by Giere and Sorensen (2004). Analyses for this study were normalized to 120, following the guidelines of Ercit (2002). 79 Allanite in the quartz veins at True Blue is Ce-dominant (Table 11). The crystals are euhedral, black, semitransparent to opaque, and prismatic with high aspect ratios. Crystal size ranges from microscopic to several em's in length (Figure 44). Figure 45 shows the relationships between allanite, clinozoisite, epidote, and ferriallanite. E M P A data plotted on this diagram indicates that F e 2 + is dominant over F e 3 + (Fe 3 + /FeTotai ~ 0.25), as defined by lines of equal oxidation state for iron. Figure 46 differentiates well the "high total" from "low total / metamict" analyses and shows that R E E are being accommodated by increased Fe and decreased Ca and A l . Chondrite-normalized R E E data from probe analyses with high totals (non-metamict) indicate L R E E enrichment with L a / G d and La/Sm values from 4.3 to 7.6 and 8.5 to 16.6, respectively. XREE2O3 values range from 18 to. 26 wt.%, T h 0 2 from 0 to 3.0 wt.%, and U and Pb are below detection limit. Although these crystals tend to contain high amounts of Th, powder X-ray diffraction patterns show that they are not significantly metamict. Allanite that does show signs of alteration and metamictization (Figure 44) exhibits low totals (as low as 81 wt.%), low £ R E E contents, high Th, and high N a when analyzed via E M P A . Alteration products include silicates, phosphates, carbonates, and oxides that are seen along fractures and along the exterior of the allanite crystals. Figure 46 shows that altered areas also have lowers-site totals and'large variations in Si , which were also been observed by Giere and Sorensen (2004). 80 TABLE 11. SELECTED ELECTRON MICROPROBE COMPOSITIONS OF ALLANITE FROM THE TRUE BLUE SHOWING Sample LAF-2 THNTP DTTH THE59 THE59 DTTH 01-4 03-2 B-l B-l l 03-3 (altered) (altered) Si02 (wt.%) 32.14 30.33 30.85 30.22 30.53 27.33 Ti0 2 0.00 0.63 0.00 0.15 0.48 0.00 Th02 0.00 0.49 2.37 0.57 1.72 3.05 A1203 19.05 14.59 17.11 15.77 15.55 18.38 Y 2 0 3 0.12 0.00 0.00 0.00 0.42 0.24 La 20 3 6.23 6.83 4.99 7.39 6.10 4.28 Ce 20 3 10.62 12.24 11.78 13.75 10.89 8.83 Pr203 0.90 1.10 1.21 1.54 1.20 0.88 Nd 20 3 3.19 4.33 3.96 4.51 4.22 3.08 Sm203 0.00 0.48 0.59 0.53 0.50 0.00 Gd 20 3 0.96 1.13 1.16 1.18 1.06_ 0.82 MgO 0.07 0.09 0.08 0.14 0.10 0.07 CaO 12.07 9.92 10.01 9.39 8.26 7.24 MnO 0.00 0.11 0.13 0.00 0.28 0.09 FeO 11.85 15.13 12.95 13.65 12.01 8.65 Na20 0.11 0.00 0.26 0.10 0.21 0.20 Total 97.29 97.39 97.47 98.89 93.52 83.14 Si 4 + (apfu) 2.899 2.903 2.893 2.870 2.980 2.880 T i 4 + 0.000 0.046 0.000 0.011 0.035 0.000 Th 4 + 0.000 0.011 0.050 0.012 0.038 0.073 A l 3 + 2.025 1.646 1.891 1.765 1.789 2.283 Y 3 + 0.006 0.000 0.000 . 0.000 0.022 0.014 La 3 + 0.207 0.241 0.173 0.259 0.219 0.166 Ce 3 + 0.351 0.429 0.404 0.478 0.389 0.340 Pr3 + 0.029 0.038 0.041 0.053 • 0.043 0.034 Nd 3 + 0.103 0.148 0.133 0.153 0.147 0.116 Sm3+ 0.000 0.016 0.19 0.017 0.017 0.000 Gd 3 + 0.029 0.036 0.036 0.037 0.034 0.029 Mg 2 + 0.009. 0.013 0:012 0.020 0.015 0.011 Ca 2 + 1.166 1.018 1.006 0.955 0.864 0.818 Mn 2 + 0.000 0.004 0.005 0.000 0.011 0.004 Fe 2 + 0.894 1.211 1.015 1.084 0.980 0.762 Na+ 0.019 0.000 0.047 0.019 0.039 0.042 Note: The following standards were used: wollastonite (SiATa, CaATa), rutile (TiKa), Th02 (ThMx), almandine (AlATa, MgATa), YAG (YLa), REE glass (LaZa, PrZp, NdLa, Sm/_p\ GdZa), Ce02 (CeZa), rhodonite (MnATa), fayalite (FeATa), and albite (NaATa). Compositions were recalculated on the basis of 12 O apfu, following the guidelines of Ercit (2002). Maximum values are shown in bold. Cr, Sr, Pb, and U were sought but not detected. 81 Figure 44. Photomicrographs (a, b) and BSE images (c, d) of allanite from the True Blue showing, (a) shows allanite in association with blue and green beryl; scale bar unit is 1cm. (b) shows several cross sections of euhedral allanite. In (c) and (d) mid tones are unaltered, dark tones are altered, and bright tones include REE-Th phosphates, oxides and (fluoro-) carbonates. Circular haloes in altered areas are due to damage from the electron beam during data collection. Fields of view in (c) and (d) are approximately 150 pm high. 82 ferr ia l lani te a l lan i te Figure 45. REE versus Al, in atoms per formula unit, for allanite from quartz veins at the True Blue showing. Lines of constant Fe 37FeT o l a l allow estimation of Fe-oxidation state. Figure after Petrik et al. (1995). Si (apfu) R E E + Fe (apfu) Figure 46. Alteration effects (a, b) and substitution mechanisms (b) for allanite within quartz veins. 83 Fluorite Fluorite is ubiquitous in the veins and ranges in colour from clear to light purple to dark purple. Crystals range in size from microscopic to clots 10 cm wide which often f i l l the width of the veins (Figure 47). N o distinct compositional zoning was seen in individual crystals using B S E mode with a S E M . The R E E content is too low to be measured via microprobe but was determined during analysis for isotopic dating and is summarized later in Table 16. Figure 48 is a chondrite-normalized R E E spider diagram for fluorite, and each sample exhibits a prominent negative Eu anomaly (~0.21 Eu*/Eu). Variations in the L R E E patterns may be due to the presence of associated LREE-enriched phases, such as allanite. Albite Albite is occasionally seen in the quartz veins and typically nucleates on existing altered feldspars (Figure 49). Crystals within the veins are unincluded, show clean albite twins, and sizes in the range of a few mm's. Oxide and sulfide minerals T i , Nb , and Fe oxides and pyrite are present in the quartz veins. The most abundant and largest of the oxides is ilmenite (FeTiOs) which typically measures ~2 x ~2 x ~0.5 mm and exhibits a platy habit. Rutile and ferrocolumbite are rare and occur either as small (~50 um long) isolated grains or with ilmenite. Compositional data is summarized in Table 9. Later alteration of Fe-bearing carbonates and allanite produced minor secondary Fe-oxides. Pyrite and chalcopyrite are rarely observed and occur as isolated fine-grained masses on the order of 0.5 cm 3 . Cathodoluminescence of minerals Cathodoluminescence (CL) of minerals in the veins has only been observed for fluorite. Figure 50 shows both C L and B S E images of a fluorite crystal extracted from the veins and mounted in epoxy. N o major tonal variations were noted for fluorite in either C L or B S E . 84 Figure 47. Photomicrographs in P P L (a) and X P L (b) of typical morphology of fluorite within quartz vein with sharp wall-rock contacts. 100 La C e Pr Nd Pm S m Eu G d Tb Dy Ho Er Tm Y b Lu Figure 48. Chondr i te-normal ized (CI) rare-earth element ( R E E ) diagram from different colours of fluorite from the True Blue showing. The normalization va lues are from McDonough & S u n (1995). Inset B S E image is of fluorite mounted in epoxy. Tonal variations are due to angled sample , not composi t ion. 85 Figure 50. S E M - B S E (a) and C L (b) images of fluorite. Tonal variations under B S E are the result of edge effects from scratches on the crystal, while tonal variations in C L may be due to minute variations in R E E concentration. 86 Fluid inclusions Fluid inclusions have been observed in quartz, beryl, and fluorite from the quartz veins. Microthermometric data was collected and interpreted by Dr. B . Linnen (University of Waterloo); however, the fluid-inclusion data is complex and w i l l only be discussed briefly here as it w i l l form the basis of a separate publication at a later date. Four types of fluid inclusions have been recognized and preliminary data can be found in Turner et al. (in review). O f the four types, Type 1 and Type 4 inclusions have been postulated to be of primary origin. These possible primary fluid inclusion populations homogenize to liquids at 139 to 238 °C and 271 to, 338 °C for types 1 and 4, respectively. Additionally, the fluid inclusions have been shown to have high salinity, up to 24 and 16 wt.% N a C l equivalent, respectively. However, one of the most significant observations is the absence of CH4 in the inclusions. This is important because the fluids at the granite-related beryl showings of Tsa da Glisza and Lened both contain C H 4 and are typically lower salinity as well . Thus, based on fluid inclusion composition, the mineralizing fluids at True Blue appear to be distinct from those of conventional beryl models. Interpretation of observations and origin of vein mass Interpretation of mineralogy and vein textures Using the system described at the beginning of this chapter, mineralized veins at True Blue can described as sigmoidal, elongate blocky, and syntaxial. Appending geochemical and mineralogical information would result in the following description: "sigmoidal elongate blocky syntaxial quartz-carbonate veins with rare-element accessory minerals". Solid and fluid inclusion bands, resorbed and multiple cores in beryl, inclusion rinds around beryl cores, elongate blocky quartz, euhedral crystals (beryl, allanite, ilmenite, and carbonate), and juxtaposed paragenetic relationships (e.g., siderite before and after beryl) are suggestive of repeated crack-seal events with open spaces into which crystals could grow. Because of the repeated nature of fluid flow and mineral precipitation and dissolution, juxtaposed paragenetic relationships are abundant. 87 Furthermore, not all vein components are present in all veins. Figure 51 is a simplified paragenetic sequence for veining at the True Blue showing. Pre-Veining Alteration Assemblage Vein Assemblage Early Mid Late biotite chlorite bastnaesite albite Fe-Ti-Nb oxides fluorite ferroan dolomite • • " ™ 1 1 1 ankerite/siderite quartz allanite beryl pyrophyllite ——m*. Figure 51 . Simplified paragenetic sequence of alteration developed prior to veining and of phases within veins. Paragenetic sequences vary b e ^ due to repeated crack-seal events arid local absence of particular minerals. 88 Emplacement conditions The shape of the sigmoidal veins suggests they were emplaced within a deformational environment undergoing shear stress. The incorporation of rotated wall rock fragments with greenschist facies assemblages indicates that the veining occurred after the production of this assemblage. Furthermore, beryl, fluorite, and carbonate minerals do not show extensive deformation textures, and thus are relatively intact. This indicates that these minerals have not been subjected to significant deformation following crystallization. This could be the result of cessation of tectonic activity or perhaps an outcome of the relatively competent and protective host rock into which the veins were emplaced. Origin of vein mass The absence of a distinct vein selvage is indicative of a system in which the fluid is either externally sourced and did not interact with the host rock or is locally sourced and is equilibrated with the host rock (Oliver and Bons 2001). The mimicry of rare-element enrichment between the veins and host syenite suggests a local fluid origin. A local source is also supported by the absence of rare-element minerals outside the syenite host rock. N o features suggestive of significant fluid migration into the system have been identified. Gradients inducing mass transfer Combinations of temperature, pressure, and chemical gradients within a defined system are required for mass transfer. These different gradients also have typical representations or indications of their relative influences. The importance of these factors are discussed by Oliver (1996) and Oliver and Bons (2001) in relation to particular vein systems. If the system at the True Blue showing is constrained to within the syenite body during regional metamorphism, then little to no temperature gradient would be present. Therefore, when the dilatant sigmoidal tension gashes were initiated in a brittle manner, a pressure gradient would be dominant, followed then by a chemical gradient (Ord and Oliver 1997). 89 Mass transfer can be achieved through diffusion and advection (Oliver and Bons 2001). Both diffusion and advection likely occurred at the True Blue showing, although overall fluid flux was likely low. Diffusion may have been the dominant form of transfer, resulting in subdued chemical gradient signatures away from the vein contact. Minor amounts of fluid likely travelled through pervasive advection through pore space and microfractures within the syenite; however large amounts of advective fluid flow have not been recognized. Hence, as the isolated tension gashes opened in a brittle manner fluid pressure inside the fracture would be lower than the surrounding rock, essentially sucking in mobile elements, primarily through diffusion (Oliver and Bons 2001). Element mobili ty and sources In order to transfer mass, it must be mobilized from an existing phase. Mobi l i ty can vary considerably for different elements under varying conditions. For example, some minerals that are considered robust under 'normal' circumstances are quickly altered and their constituents are mobilized under the appropriate conditions. Presumably, the syenite host for the quartz veins had undergone complete solidification before regional compressional deformation. I f the vein mass is constrained as being local, all elements present in the vein must have therefore originated from phases within the syenite. Common elements Quartz and carbonate minerals are the dominant phases in veins. Their solubility has been well studied (e.g., Fein and Walther 1989, Manning 1994, Newton and Manning 2002, Caciagli and Manning 2003, Newton and Manning 2003, and references therein for each). The solubility of quartz, in general, is positively correlated with salinity, temperature and pressure. The solubility of carbonate is more variable. A t upper crustal temperatures and pressures, solubility increases with increasing pressure, but decreases with increasing temperature (Fein and Walther 1989). A t lower crustal temperatures and pressures, solubility increases with temperature, pressure, and salinity (Newton and 90 Manning 2003). Si and CO3 2 " are common constituents in a variety of vein systems and the aforementioned studies describe their overall high solubility. Silica is perhaps the most difficult constituent to account for in the veins at the True Blue showing. Although syenite is typically considered to contain no modal quartz, intrusions can be locally quartz saturated. Two other factors increase the availability of Si : (1) Silicification of the intrusion may have occurred through syn-emplacement hydrothermal cells, and (2) Si may have been released during prograde or retrograde metamorphic reactions through the destruction of primary ferromagnesian minerals or in association with the development of flame perthite. Carbonate in the veins is easily accounted for by the ubiquitous secondary carbonate in the syenite. Less abundant than quartz and carbonate are beryl, allanite, and fluorite. Aluminum in beryl, Ca and A l in allanite and Ca in fluorite can be sourced from destroyed primary alumnosilicates or secondary minerals such as albite or annite. Fluorine is commonly enriched in syenite magmas and is often sequestered into fluorite or substituted as a trace element for OFT in a variety of hydrous minerals. Rare elements (REE + Be + Ti) Although allanite, beryl, and ilmenite do not comprise the bulk of the vein mass, their presence is indicative of an anomalous amount of R E E s , Be, and T i within the system, respectively. The group of rare earth elements is typically considered to be immobile during alteration events. Consequently, these elements are often used as geochemical indicators representative of former geochemical affinities of the particular rock. A number of . studies have shown that these elements can be highly mobile depending on conditions (e.g., Wood 1990a, Haas et al. 1995, and Bau 1996) and mobility is investigated either by aqueous speciation (Wood 1990a, 1990b, 2003) or by changes in whole-rock geochemistry (Rolland et al. 2003). Wood (2003) provided extensive information on the stabilities of many complexing agents at a variety of temperatures. A s a very general statement, fluoro-carbonate complexes appear to be effective agents for mobilizing and transporting REEs , although CI and PO4 complexes have also been shown to be significant agents. Rolland et al. (2003) investigated R E E mobility in mid-crustal shear 91 zones and concluded that element enrichments in syntectonic veins are most strongly correlated with geochemical affinity of the wallrock. Secondary to this, and perhaps a circular argument, is correlation with ligand availability. For instance, an element that is highly concentrated in a rock subjected to alteration may remain immobile if no complexing agent is available. In two mineral deposit studies where allanite is present, the concentration of REEs in the fluid phase was determined to reach up to 1290 ppm REE-Totai (Banks et al. 1994, Giere 1996). Studies by Forster (2000, 2001) indicate that REE were mobilized over distances of decimetres by F-CI-CO3 complexing agents after monazite-xenotime breakdown during late-stage autometasomatic alteration within granite. The whole-rock geochemistry of the syenite at the True Blue showing exhibits enrichment in REE, suggesting that primary or syn-emplacement alteration minerals {e.g., monazite, xenotime, zircon, and bastnaesite) with essential REE were likely present. Thus, these phases were likely the source of REE for allanite in the quartz veins. However, minor amounts of these rare-elements may have also been sourced from destroyed primary phases with trace amounts of REE. Possible destroyed primary silicate minerals listed in section 4.2.3 that can incorporate significant amounts of REEs include the amphibole and pyroxene minerals. However, the presence of metamorphic monazite and bastnaesite alongside altered zircon suggests that the REE were dominantly sourced from phases with essential REE. Somewhat paralleling the studies on REE mobility are studies concerning the mobility of Be although the number and scope of these studies is much smaller. More recently several papers have dealt with Be in magmatic and metamorphic environments, however the last pertinent papers on aqueous geochemistry of Be were put forth by Wood (1992) and Franz and Morteani (1981). A recent review of the element Be was recently published (Grew 2002b) and provides information pertaining to the mineralogy, petrology and geochemistry of this light element. It is of particular note that most beryl and non-beryl Be deposits have an association with F-bearing minerals (Barton and Young 2002). From this alone it can be hypothesized that F-related complexes likely have a prominent role in the transport of Be. Wood (1992) investigated Be solubility in aqueous environments in the presence of F and a number of other possible carriers, such 92 as CO3 C l r , OH" and SO4 ". Conclusions from that study suggest that F , CO3, O H , F-CO3 and F - O H complexes are important, however F-CO3 is the dominant complex at temperatures above 200 °C and at p H levels between 5 and 7. The exact source of Be in the syenite host rock at the True Blue showing is unclear. N o primary minerals with essential Be have been noted from the syenite at True Blue, however phenakite, bertrandite, gadolinite and helvite have all been recorded as primary igneous phases in a number of peralkaline quartz saturated and undersaturated intrusions (Barton and Young 2002). Relevant phases with significant but non-essential Be include the feldspar and amphibole minerals and phyllosilicates, which have been known to carry up to -25 ppm Be each (Grew 2002a). Li-bearing phyllosilicates are an exception and Be contents up to -500 ppm have been reported (Grew 2002b). Although 25 ppm appears to be low, the formation of beryl deposits does not require leaching of strongly enriched host rocks provided that enough 'normal' host rock is altered (Giuliani et al. 1999). Thus it is possible that Be was sourced from phases with or without essential Be. Further delineating the source of Be in the syenite host rock is time consuming and expensive. The major hindrance for this is the inability of conventional E M P techniques to detect and quantify concentrations of Be, among other light elements. Elevated T i is exhibited in the veins through ilmenite and ferrocolumbite, as well as Nb-rutile. Titanium is relatively insoluble in H 2 0 but has been reported to have higher solubilities in the presence of CO3 ~, F", and C f (van Baalen 1993, Wiegand and Seward 1997, Widmer and Thompson 2001, Tropper and Manning 2005). The source of T i may have been primary ferromagnesian minerals but was more likely to have been primary T i oxide minerals and mixed Ti-Nb-Fe oxide minerals. A s summarized above, the mobility of these typically immobile elements was enhanced dominantly through complexation with F", CO3 2", and mixed F - C O 3 2 and secondarily through complexation with OH" and Cl" . These are now represented in the tension gashes primarily by fluorite and carbonates. The majority of the exotic elements included in allanite and Ti -Nb oxide minerals likely resided in primary or syn-emplacement alteration minerals with these elements as essential constituents of the minerals. Beryll ium could be attributed to a number of minerals and likely existed as a trace element in secondary minerals. 93 Mechanisms of precipitation Once present in the veins, aqueous complexes require a change in conditions to promote crystal nucleation and growth. Common mechanisms for the initiation of crystallization in hydrothermal conditions are fluid mixing, fluid-rock interaction, temperature changes, and pressure changes. Fluid mixing is unlikely to be a major consideration at True Blue since the system is likely to be generally closed. However, sub mm-sized veins indicate that minor communication between vein sets may have occurred, and thus by extension there may have been communication with an external fluid. Precipitation induced through fluid-rock interaction is also unlikely since the fluid is locally sourced and the two have been shown to be largely in equilibrium. This leaves change in pressure and temperature as the primary causes of mineral precipitation. Changes in these parameters may have also induced dissolution of mineral phases during reactivation of the tension gashes. This is well documented in beryl where anhedral cores are common. Summary In summary, minor external fluid infiltration may have been present; however, the system is perhaps best described as "closed system - fractured" as per Oliver (1996). These systems are described as ones in which the fluid is equilibrated with the host rock, the vein mass is dominantly locally sourced (from millimetre to metre scale), and there is commonly repeated fracturing with restricted interconnectivity. Mineralized veins at True Blue can be described as "sigmoidal elongate blocky syntaxial quartz-carbonate-fluorite veins with rare :element accessory minerals". Lack of deformation of the mineralization, dilational sigmoid shape, and capture of greenschist facies wallrock suggest the veining occurred late in or following peak metamorphism and tectonism. Rare-element mimicry between the vein and wallrock, capture of altered wall rock within the vein, and lack of distinct vein selvages together suggest a local source for the mass within the veins. Transfer of this mass into the veins was driven primarily through pressure differences and secondarily through chemical gradients. Exotic elements (e.g., T i , N b and R E E ) were likely sourced from secondary minerals where they were essential constituents and Be was likely sourced from secondary minerals where it 94 resided as a trace element. These rare elements were mobilized dominantly by mixed F~-CO3 " complexes, which are now represented by fluorite and ubiquitous carbonate minerals. Precipitation was likely induced by decreases in temperature and pressure and stable open cavities allowed the formation of euhedral crystals. 95 C H A P T E R 6 S T A B L E ISOTOPES Purpose Stable oxygen isotope studies were undertaken primarily to better resolve the temperature of vein formation at the True Blue showing, and secondarily to characterize the isotopic composition of the fluid. Oxygen isotope data were collected from mineral separates of quartz (n = 8), beryl (« = 1), and carbonate minerals (n = 4) from the veins. In situ stable oxygen-isotope data (27 points) were collected from six beryl crystals. Carbon isotope data were also collected on the carbonates. Multiple crack-seal and fluid flow events and apparent disequilibria between sampled whole-mineral phases do not allow direct determination of fluid temperature. However, combining data from the mineral separates and in situ analyses allows for some general constraints to be drawn. Also , i f the fluids are dominantly locally derived the isotopic composition of the fluid should reflect that of the host rock (Oliver 1996, Oliver and Bons 2001). Methodology Analytical procedures for whole-mineral and in situ isotopic data collection can be found in Appendix 1. Data In situ In situ 5 OSMOW values (n = 27) for the beryl range from +1.7 to +9.0%o, and variations within a single crystal are up to 6.0%o. There are two populations of in situ 8 1 8 OSMOW data that can be divided into a heavier range of 5.5-9.0%o and lighter range of 1.7-4.45%o. Most crystals analyzed have cores visible in B S E images (Figure 52) but the 18 compositional zoning does not correspond to 5 OSMOW patterns between samples, such as heavy rims or cores. Data is summarized in Table 12. 96 Figure 52. B S E images of beryl crystals used for in-situ stable oxygen isotope analyses Circles indicate spot locations and values indicate 5 1 8 0 S M O W obtained. Cores with lighter atomic mass (darker tone) are visible in most crystals. All crystals have been mounted perpendicular to the c-axis with the exception of one samples in (e) from 46c. 97 T A B L E 12. IN-SITU 5 1 8 0 S M o w D A T A OF B E R Y L F R O M T H E T R U E B L U E S H O W I N G Sample 8 1 8 OSMOW E r r o r Normalized In (%o) (%o) Position on Crystal Core GS2a 6 0.25 0.36 2.5 0.15 0.38' 7.4 0.3 0.64 * 8.5 0.2 0.85 6.8 . 0.2 0.93 GS2b 5.6 0.2 0.29 6.5 0.1 0.60 8.5 0.2 0.75 7.4 0.2 0.76 6.1 0.2 0.92 46a (small) 7.6 0.3 0.12 6 0.15 0.53 4.45 0.2 0.73 46a (big) 6.8 0.3 0.26 * 5.8 0.1 0.53 3.4 0.2 0.68 2.1 0.2 0.93 46c (cross) . 6.75 0.2 0.10 6.85 0.15 0.45 * 1.7 0.2 0.92 2.3 0.2 0.92 46c (vert) 8.7 0.15 0.45 * 6.4 0.2 0.81 7 0.2 0.97 * 46b 5.5 0.2 0.09 9 0.1 0.50 6.4 0.15 0.92 98 5 Whole-mineral Oxygen to Whole-mineral 8 OSMOW values for quartz («=8, Table 13) cluster around 11.1 %o, range from 9.2 to 12.1%o, and average 10.8%o. The whole-mineral 5 OSMOW value for beryl is 7.3%o, which is intermediate in the range of in situ 5 1 8 O S M O W values for beryl. Two samples of siderite from different veins both gave values of 11.8%o, while a sample of dolomite gave a value of 9.4%0. A third sample of siderite gave a value of 13.9%o, however the signal at the detector was reported as being low, suggesting the phase was impure. Carbon Whole-mineral 8 1 3 C P D B values for carbonates are given in Table 13. The lowest value of -1.7%o came from one sample of siderite; however, the signal at the detector was reported to be small and was likely due to an impurity (e.g., alteration). Consequently, the range of reliable values for 8 C P D B from carbonates is-3.5 to-5%o. Interpretation of oxygen isotopes Whole-mineral S 1 8 O S M O W pairs between quartz, beryl, and carbonate do not result in consistent temperatures. Only two paired samples resulted in computable temperatures of 595 and 767 °C for calcite-beryl and quartz-beryl, respectively. Other paired samples result in temperatures exceeding 1000 °C, and mineral pairs between samples typically result in temperatures of 450 °C and up or 150 °C and below. This indicates that minerals that were thought to have co-precipitated did not, isotopic exchange occurred after precipitation, and/or minerals are strongly isotopically zoned. B y pairing in situ oxygen isotope data from beryl with whole-mineral analyses from quartz, a range of possible fluid temperatures for the vein can be calculated. This study used fractionation factors between quartz-H 2 0 and beryl-FI^O from Zheng (1993) since they are from one consistent source, however other reliable fractionation factors do exist (e.g., Bottinga and Javoy 1973, Clayton et al. 1972, Matsuhisa et al. 1979). A s a cautionary note, there may be inherent error associated with this approach. Most significant is the fact that the in situ data came from liberated crystals and the whole-99 T A B L E 13. W H O L E M I N E R A L 5 1 3 C P D B A N D 5 1 8 0 S M o w V A L U E S F R O M S E L E C T E D P H A S E S W I T H I N M I N E R A L I Z E D V E I N S Sample Mineral 8 1 3 C p D B 8 1 8 OSMOW E X T - 4 6 calcite -5 9.4 E X T - 4 8 siderite . -3.8 11.8 T H A - 4 siderite . -1-7 13.9 THII-B siderite -3.5 11.8 E X T - 4 6 beryl 7.3 E X T - 4 6 quartz 9.2 TH-11B quartz 11.4 T H A - 4 quartz 10.8 T H E 5 9 A quartz 11.0 T H E 5 9 B quartz 10.7 T H E 5 9 C quartz > 10.8 TH03 quartz 12.1 THNTP01 quartz 10.7 100 mineral data came from quartz that is not directly associated with the beryl crystals, but likely precipitated from fluid of the same source. Also , whole-mineral data w i l l average 18 any variations in 8 OSMOW values, which are evidently present in at least the beryl crystals (Figure 52). The heavier population of beryl in situ data is less than but close to the value for whole-mineral quartz, thus requiring less fractionation and therefore indicating a higher temperature. The opposite is true for the lighter population whose 5 1 8 O S M O W values are further from quartz. Figure 53 shows the results for temperature calculations between quartz-beryl pairs. Dashed lines indicate isotherms defined by specific A q t z - b r i values, and the number of calculated values in each range is also given. The heavy population's maximum and minimum A q t z - b r i values are 6.6 and 0.2%o, which correspond to temperatures of 196 and 1902 °C, respectively. The light population's maximum and minimum A q t z . b r i values are 10.4 and 4.75%o, which correspond to temperatures of 69 and 315 °C, respectively. If all the possibilities between in situ data from beryl and whole-mineral data from quartz are considered, the majority of the A q t z - b r i values concentrate between 3.9 and 5.3%o. This suggests fluid temperatures between -425 and -275 °C, respectively The extreme values (e.g., 69 and 1902 °C) are not consistent with mineral assemblages of the veins or host rock, however, the dominant range of temperatures (between -275 and -425 °C) is consistent with the observed assemblage as wel l as 1 R greenschist facies conditions. These fluid temperatures also suggest 8 OSMOW fluid compositions, assuming pure H2O, in the range of -2 to 7%o, which covers many possible fluid reservoirs, according to data from Rollinson (1993) and references therein. In view of the range of fluid compositions obtained through fluid inclusion studies (up to 24 wt.% N a C l equivalent), this must be considered with caution. Some possible explanations, other than changing fluid conditions, for the anomalously high and low in situ 8 1 8 O S M O W values (i.e. ~2%o and ~9%o), and resulting anomalous temperatures (below 110 and above 650 °C), include either a coexisting light 18 or heavy 8 OSMOW phase in contact with the beryl at time of crystallization. Unfortunately the crystals from which the in situ measurements were collected originated from the bulk samples and were obtained through crushing and liberation and as such, no 101 e>°J° 10 I O a To 8 „r\°G-CD 4 2H 0 LO|0O-O O o 8 8 o o o o o o-OO" o fl fl fl 88 8 88^8" o -fl 8 o o o fl 8 8 >°G- High Population o ^ - ITow Population f.C-,+6 G - O O O o o ^<r o o o o o o o o o o o o oo, ^ o" o „o -- -o o o o o In-situ beryl • Whole mineral beryl ,6° '° 8.0 9.5 11.0 Quartz (5180SMOW%o) 12.5 Figure 53. Stable oxygen isotope pairs from in situ and whole mineral beryl analyses combined with whole mineral quartz analyses. Dashed lines indicate isotherms, as defined by fractionation factors from Zheng (1993). Total number of data points is 224 (8 quartz * 27 + 1 beryl). Repeating values occur for quartz at 10.7 and 10.8 and for beryl at 6.0, 6.4, 6.8, 7.4, and 8.5. Values to the right and above the diagram indicate the number of samples within that range and their percentage of the sample suite. 102 in situ evidence for coexisting phases exists. Interpretation of carbon isotopes Since no two analyzed carbonate phases coprecipitated, information extractable from 8 1 3 C P D B isotopes includes an isotopic signature of the fluid. This can be achieved through back-calculation of the fluid composition using the estimated temperature obtained through oxygen isotope calculations (from -275 to 425 ° C ) . Values of 8 1 3 C P D B for the fluid derived from the above temperatures range from -2.32%o to -7.49%o and average —4.9%o. This range of values is consistent with a number of defined reservoirs for carbon, including mantle, marine carbonate, metamorphic and groundwater sources (Faure 1986, Rollinson 1993, and references therein). Summary In situ and whole-mineral stable oxygen isotope data supports precipitation of constituents in the veins between -275 and -425 ° C . However, data variation from in situ techniques shows that at least one mineral is isotopically heterogeneous (beryl). This heterogeneity is not satisfactorily explainable via temperature variations, growth periods, compositional variations, or sector zoning. 103 C H A P T E R 7 G E O C H R O N O L O G Y Introduction The origin and timing of fluids associated with beryl mineralization at the True Blue showing is not definitive from field relationships. Consequently, constraining the timing of this event using geochronology is important for understanding the origin of the mineralizing fluids. Initial hypotheses regarding the formation of beryl at the True Blue showing and the origin of the fluids, like most gem-beryl deposit models used in the northern Cordillera, involved magmatic fluids. Two separate magmatic events had been considered, one at -100 M a and the other at -360 M a , however an alternate source and timing for the fluids had not been ruled out. Dating of mineral deposits can be approached indirectly by dating events prior to and after mineralization, or it can be approached directly by dating one of the phases involved in mineralization. Inspection o f the vein mineralogy at True Blue, and accordingly the mineralizing event, revealed two phases that had been previously used with success in other geochronological studies: fluorite and allanite (e.g., Chesley et al. 1991, Barth et al. 1994). Consequently, direct dating of the mineralizing event was potentially possible. Geochronological studies of fluorite found in the literature primarily used the radiogenic isotope pair Sm-Nd, while allanite generally attracted the attention of U-Pb techniques. Fluorite was investigated first because of its optical and chemical homogeneity, the relatively high R E E values observed in whole-rock analyses of the syenite, and successfulness of previous studies. Subsequent to obtaining the Sm-Nd geochronological results U-Pb studies were undertaken on allanite. Accessory mineral dating A brief discussion of geochronological concerns is appropriate. Two main assumptions must be made when attempting to determine an age from a particular isochron. First, it must be assumed that at t = 0 the system was isotopically homogenous. 104 Second, it must be assumed that the system in question has remained isotopically closed since t = 0 and consequently that no mass transfer of either parent or daughter element has occurred across the sample suite. In addition to these requirements, two other factors can increase the accuracy and reliability of an isochron. These are factors are (1) a large amount of parent element compared to daughter element and (2) a large range in parent/daughter ratio. Together, these factors allow a more.accurate determination of isotopic ratios and a well 'stretched' isochron, thereby lowering the effective age error from the isochron. Previous studies Fluorite has been used in several studies to date hydrothermal mineralization in which it occurs as a gangue or ore mineral (e.g., Turner et al. 2003, Chesley et al. 1991,. 1994, Munoz et al. 2005). Fluorite can incorporate moderate amounts of R E E s (Deer et al. 1992) and studies by Cherniak et al. (2001) indicate diffusion of these elements is very slow at moderate temperatures (~500 °C). Coupled with its abundance in many mineralized systems, fluorite can be a precise and powerful Sm-Nd geochronometer despite the long half-life of 1 4 7 S m (6.54 x 10"1 2 y 1 ) . A common observation in the above studies is that a range in colour generally correlates to a range in R E E concentrations and Sm/Nd ratios. Consequently, a large spread in 1 4 7 S m / 1 4 4 N d values and thus relatively precise age determinations are often possible from a monomineralic study of fluorite. Recently allanite has been touted as an important accessory mineral in many geologic environments (e.g., Barth et al. 1989, Barth et al. 1994, Davis 1994, Oberli 1999, Catlos 2000, Giere and Sorensen 2004). Its ability to incorporate radiogenic elements (e.g., Sm, Rb, U , Th), wide range of element substitutions, and wide range of stability enable allanite to record much about its varied environments. Common'problems encountered when using allanite as a geochronometer are metamictization, alteration, high common Pb, and chemical inheritance of radiogenic Pb from a previously consumed phase (e.g., monazite, titanite or zircon) (Romer 2003, Giere and Sorensen 2004). This last concern is most significant in regional metamorphic rocks where allanite has grown in situ at the expense of another mineral. 105 Geochronology of the Pelly Mountains Selected radiogenic ages from the YukonAge database (Breitsprecher et al. 2003) for locations in the Pelly Mountains that are relevant to deciphering mineralization at True Blue can be divided into three groups: igneous-related Mississippian ages, metamorphic-related Jurassic ages, and igneous-related Cretaceous ages (Table 14, Figure 54). The Mississippian ages reflect igneous activity related to the emplacement of syenites (unit M y ) and their extrusive equivalents (unit Mva) . A n U-Pb date from zircon in syenite gave a well defined age of 362.7 ± 3.6 M a (Breitsprecher et al. 2003, J .K. Mortensen, pers. comm.); however, K - A r and Rb-Sr analyses from a REE-skarn in contact with the same syenite stock yielded ages of 326 ± 20 M a and 333 ± 20 M a , respectively (Chronic 1979). Jurassic ages of 160 ± 10 M a (Chronic 1979), 189 ± 22 M a (Wanless et al. 1979), and 190.4 ± 1.6 M a (Fallas et al. 1999) were obtained by K - A r methods on one biotite and two whole-rock samples, respectively. The two younger ages (-160 M a and -189 Ma) were obtained from igneous rocks likely of Mississippian age, and the older age (-190 Ma) was obtained from phyllite of the Paleozoic Cassiar Terrane in the footwall of the St. Cyr klippe (Figure 9). These ages are likely the result of resetting during regional metamorphism and deformation. Many wel l defined K - A r , Rb-Sr, and U-Pb ages from whole-rock samples and a variety of minerals record widespread Cretaceous (-110 Ma) igneous activity in the region (e.g., Wanless et al. 1979, Mortensen and Hansen 1992, Fonseca 1998). Sm-Nd geochronology of fluorite Methodology Six samples of optically clear and unzoned, purple to colourless fluorite were hand picked from four different quartz veins within "Shark B o w l " and utilized for Sm-Nd geochronology. Complete techniques for acquisition of data are described in Appendix 1. Fluorite from the veins was chosen as the prime phase for geochronology because of its direct association with beryl, range of colours, and lack of inclusions or zoning, as well as the presence of the REE-enriched host rock. Consequently, a wide range of , 4 7 S m / 1 4 4 N d was probable. 106 T A B L E 14. S E L E C T E D G E O C H R O N O L O G I C A L D A T A F R O M Y U K O N A G E D A T A B A S E (Breitsprecher et al. 2003) Age Error Interpretation Rock Type Rock Description Lat/Long (Nad27) NTS • Method Reference, YukonAge Ref 362.7 3.6 Igneous Age Plutonic Mississippian quartz syenite stock 61.5N 132.46W 105F/9 U/Pb zircon Mortensen unpublished, see Breitsprecher et al 2003, 117 333 20 Mineralization Age Hydrothermal Skarn adjacent to Mississippian syenite 61.49N 132.41 W 105F/8 Rb/Sr w.r.-mineral Chronic 1979,9 326 20 Age of Alteration Hydrothermal Skarn adjacent to Mississippian syenite 61.49N 132.41W 105F/8 K/Ar phlogopite Chronic 1979, 9 304 38 Geological Error Volcanic Altered and metamorphosed trachyte and syenite 61.45N 132.66W 105F/7 Rb/Sr whole rock Mortensen 1982, 56 294' 40 Geological Error Volcanic Altered and metamorphosed trachyte and syenite 61.45N 132.66W 105F/7 Rb/Sr whole rock Mortensen 1982, 56 190.4 1.6 Reset Hydrothermal Phyllite 61.08N 132.36W 105F/1 Ar/Ar whole rock Fallas etal 1999, 175 189 22 Metamorphic Cooling Volcanic Felsic tuff; protolith age is Devono-Mississippian 61.55N 132.56W 105F/10 K/Ar whole rock Wanless et al 1979, 105 160 10 Cooling 300°C Plutonic Mississippian syenite stock 61.51N 132.44W 105F/9 K/Ar biotite Chronic 1979, 9 128 50 Metamorphic Age Plutonic Mississippian syenite stock 61.5N 132.44W 105F/9 Rb/Sr w.r.-mineral Chronic 1979, 9 125.3 1 Geological Error Hydrothermal Manto-style mineralization at Ketza River mine 61.53N 132.28W 105F/9 Ar/Ar biotite Fonseca 1998, 188 113 8 Cooling 500°C Plutonic Mafic dyke 61.5N 132.44W 105F/9 K/Ar hornblende Chronic" 1979, 9 112 4 Cooling 300°C Plutonic Lamprophyre minette dyke 61.54N 132.58W 105F/10 K/Ar biotite Stevens etal 1982, 91 108 0.6 Mineralization Age Hydrothermal Quartz-sulphide vein at Ketza River mine 61.54N 132.26W 105F/9 Ar/Ar muscovite Fonseca 1998,188 103.2 2.7 Cooling 300°C Plutonic Mafic dyke near northern margin of Seagull Uplift 61.64N 132.81W 105F/10 K/Ar biotite Hunt and Roddick 1991, 32 102.7 1.6 Cooling 350°C Plutonic Granite associated with the Risby W deposit 61.85N 133.38W 105F/14 K/Ar muscovite Hunt and Roddick 1987, 29 101 8 Metamorphic Cooling Metamorphic Hornfels biotite hornfels at Ketza River mine 61.53N 132.29W 105F/9 K/Ar whole rock Armstrong and Dawson unpublished, see Breitsprecher et al 2003, 110 101 1.8 Cooling 350°C Plutonic Muscovite granite near centre of Seagull Uplift 61.55N 132.68W 105F/10 K/Ar muscovite Hunt and Roddick 1991,32 100 1 Metamorphic Cooling Plutonic Granodiorite gneiss overprinted by Nisutlin Batholith 61.56N 133.07W 105F/11 Ar/Ar biotite Hunt and Roddick 1992, 34 " 99.2 3.6 Cooling 300°C Plutonic Quartz monzonite (White Creek Stock) 61.43N 132.18W 105F/8 K/Ar biotite Wanless et al 1979, 105 98.2 1.5 Cooling 350°C Plutonic Granite associated with the Risby W deposit 61.85N (33.38W 105F/14 K/Ar muscovite Hunt and Roddick 1987,29 96 3.5 Cooling 300°C Plutonic Quartz monzonite (Nisutlin Batholith) 61.39N 132.6W 105F/7 K/Ar biotite Wanless et al 1979, 105 94 5 Cooling 300°C Metamorphic Quartz-feldspar-hornblende-biotite schist 61.66N 133.33W 105F/11 K/Ar biotite Wanless et al 1967,98 92 .6 Cooling 300°C Plutonic Quartz monzonite 61.59N 133.44W 105F/11 K/Ar biotite Wanless et al 1967, 98 92 6 Cooling 300°C Plutonic Granite (Big Salmon Batholith) 61.76N 133.44W 105F/14 K/Ar biotite Wanless et al 1967, 98 91.2 3.3 Cooling 300°C Plutonic Quartz monzonite (Nisutlin Batholith) 61.48N 132.81W 105F/7 K/Ar biotite Wanless et al 1979, 105 86.4 3.2 Cooling 300°C Plutonic Quartz monzonite (Nisutlin Batholith) 61.25N 132.84W 105F/7 K/Ar biotite Wanless etal 1979,105 86 26 Cooling 500°C Metamorphic Gneiss horablende-plagioclase gneiss 61.68N 133.26W 105F/11 K/Ar hornblende Wanless et al 1967, 98 Paleozoic platform sedimentary rocks ["1 Mississippian metavolcanic rocks (Pelly Mountain Volcanic Belt) Mississippian syenite Cretaceous granitoids Major faults YukonAge data locations and ages in Ma • 10 Km Figure 54. Simplif ied geological map of the Pelly Mountains showing the approximate location and calculated age (in Ma) of geochronological samples listed in YukonAge (Breitsprecher et al. 2003). Teeth on thrust faults indicate upper plate. Data plotted using U T M Zone 9N and N A D 8 3 . S e e Figure 9 for geographic labels. Data Sm-Nd isotopic data are presented in Table 15. Sm and N d concentrations show ranges from 0.32-1.36 ppm and 0.28-3.26 ppm, respectively, to ta l R E E values range from 6.87 ppm to 41.48 ppm. 1 4 7 S m / 1 4 4 N d and 1 4 3 N d / 1 4 4 N d values show a wide range from 0.22-0.68 and 0.512620-0.513130, respectively. E u N / E u N * values range from 0.12 to 0.27, indicating a negative E u anomaly for each sample. Increasing saturation of colour (purple) roughly corresponds to decreasing R E E concentrations and higher 1 4 7 S m / 1 4 4 N d values. Interpretation Data was plotted and an isochron was fit using the program Isoplot 3.0 (Ludwig 2003). This software uses regression algorithms of York (1969) and uses a simple "Model 1" fit for determining Sm-Nd isochrons. This "Model 1" fit assumes that scatter is a result of only the inputted error, which is determined during data collection. Figure 55 shows the calculated line, which yields an age of 171.4 M a ± 4.8 M a ( M S W D = 0.26) with an sNdcHUR value of-1.10 and intercept of 0.51236. There is little is scatter from the calculated line, resulting in the small age error. This mid-Jurassic age of fluorite crystallization coincides with timing of regional compressional tectonic activity as deduced through geological field observations (Templeman-Kluit 1979). Thus, it supports the idea that the beryl mineralization at the True Blue showing is associated with syn-tectonic fluids and argues against the idea that mineralization is tied to igneous activity. The chondrite-normalized R E E spidergram (Figure 48) for fluorite shows a range of patterns that are suggestive of a coprecipitating LREE-enriched phase for the samples with depressed L R E E s . A natural and likely candidate is allanite, which has been shown to be L R E E enriched (Chapter 5). 109 TABLE 15. SM-ND GEOCHRONOLOGICAL DATA FOR FLUORITE FROM THE TRUE BLUE SHOWING Sample Colour Sm (ppm) Nd (ppm) 1 4 7 Sm/ 1 4 4 Nd 1.5% error 1 4 3 Nd/ 1 4 4 Nd 2o DT54CA Clear 0.63 1.61 0.2360 0.0035 0.512620 0.000007 BWSHD4 Light purple 0.44 0.84 0.3140 0.0047 0.512706 0.000007 DT36 Light purple 0.80 2.19 0.2209 0.0033 0.512598 0.000015 DTTH04 Light purple 1.36 3.26 0.2524 0.0038 0.512641 0.000007 DT54PA Medium purple 0.38 0.40 0.5793 0.0087 0.513004 0.000007 DT54PBS Dark purple 0.32 0.28 0.6882 0.0103 . 0.513130 0.000018 0.5133 0.5131 X3 z 0.5129 T3 2 0.5127 0.5125 eNd = -1.10 MSWD = 0.26 Medium and dark SZs/ purple y//' Light purple and clear 0.1 0.3 0.5 147Sm/ 144Nd 0.7 Figure 55. Sm-Nd isochron from fluorite in quartz veins at the True Blue showing. The size of the circles is equal to or greater than the error. MSWD = Mean Square of Weighted Deviates. U -Pb geochronology of allanite Geochronological studies were undertaken on euhedral allanite from beryl-bearing quartz veins. A description of sample preparation and data acquisition via ID-T I M S techniques can be found in Appendix 1. The results from this monomineralic 110 approach were inconclusive and the isotope systematics of allanite from the True Blue showing are not completely understood at this time. Additional studies using in situ techniques are underway. Geochronological data can be found in Appendix 5. Summary A well defined mid-Jurassic age was obtained from fluorite; however this age challenges conventional exploration models for gem beryl in Yukon. Dating was attempted on allanite using U-Pb ID-TIMS methods, however additional in situ geochronological work is required to fully understand the significance of that data. Furthermore, five samples of secondary monazite retrieved from crushed whole rock are slated for analysis in the near future. Confidence of data collected in other studies in the Pelly Mountains is generated through reproducibility of age ranges, their correlation with regional events, and reported quality in the YukonAge database and original publications. Confidence of the Sm-Nd age obtained during this study is generated by quality of analytical procedures, resulting precise data, correlation with other evidence (local and regional) and isotopic systematics of fluorite. To summarize, timing of mineralization has been dated directly through Sm-Nd geochronological studies on fluorite that precipitated with beryl and timing of regional events has been bracketed by numerous previous geochronological studies. A s interpreted by geochronological data, a sequence of events in the Pelly Mountains can be described: 1- Mississippian crystallization of syenite 2- Mid-Jurassic regional thermal resetting of sedimentary and volcanic rocks 3- Mid-Jurassic local fluid flow and crystallization 4- Mid-Cretaceous crystallization of granitoids The mineralization at the True Blue showing is the first mineralization from the mid-Jurassic to be conclusively dated in the Pelly Mountains. Accordingly, this delineates a previously under-documented period of mineralization in the Pelly Mountains, southern Yukon Territory. I l l C H A P T E R 8 D E P O S I T M O D E L Introduction Gem beryl deposits^are best understood by first identifying the source of Be. Phases co-precipitating with beryl are worthy of investigation because the source of elements in those minerals may be the same as for beryl. The means by which Be is transported is also important in developing genetic and exploration models for beryl mineralization since the nature and signature of the fluid can give clues to the origin of the mineralization. A t the True Blue showing a notable accessory mineral is allanite. This mineral contains essential REEs , which have similar complexing agents as Be (e.g., fluoro-carbonates), thus making it a valuable proxy mineral. The source of chromophores in beryl can also assist in developing a deposit model; however the chromophore for beryl at the True Blue showing (Fe) can be easily sourced from either the syenite or the Fe-rich stratigraphy adjacent to the host rock. Deposit model Bearing in mind local and regional geological information and current popular gem beryl deposit models for Yukon Territory, there are three possible Be sources and three corresponding ages of mineralization for the True Blue beryl showing: (1) Be-bearing magmatic fluids released from a buried Cretaceous granite (circa 100 Ma) (2) Be-bearing magmatic fluids released from the Mississippian syenite (circa 360 Ma) (3) Be leached from solidified Mississippian syenite (post Late Triassic - pre Cretaceous) Current deposit models for gem beryl mineralization in Y u k o n Territory require an evolved pluton that exsolves late-stage Be-rich fluids into surrounding country rock. Suites of intrusions that satisfy this situation typically have Cretaceous ages. There are no mapped Cretaceous granites within the vicinity of the True Blue showing; however the likely presence of a buried granitoid at nearby Ketza River gold mine raises the 112 possibility of a similar circumstance at True Blue. Three main problems arise with this model. First, the geochemical affinity of the mineralizing fluid is not consistent with late-stage granitic fluid (e.g., no enrichment in B , M o , Sn, W and C 0 2 » C H 4 ) . Second, the shape and spatial restriction of mineralized veins are not consistent with fluids originating from a Cretaceous granitoid. Third, geochronological data does not correlate with known igneous activity within the Pelly Mountains. If the 'Cretaceous granitoid' model is slightly modified to include intrusions of older age and different classification, a 'Mississippian syenite' model can be constructed with many of the same features. Since whole-rock geochemical values for the syenite show enrichment in R E E and Be, this could allow for a 'simple' closed system. If mineralization did occur during the waning stages of syenite emplacement, deformation temporally close to complete solidification of the magma might produce the dilatant tension gashes and a place for residual Be and R E E rich fluids to escape. However, the solid inclusions within the veins that show greenschist facies alteration assemblages would have had to be produced prior to veining. Furthermore, the geochronological data do not correlate with this model. The third and best-supported model involves Be leached from the syenite host rock into metamorphic fluids during regional tectonism (Figure 58). Similar to the Mississippian 'igneous' hypothesis, enrichment in the rare-elements (Be, REEs , T i , Nb , etc.) is present from the intrusion thereby eliminating the need for an external source. Additional supporting evidence for the local derivation of vein components includes the restriction of beryl, allanite, and Fe-Ti-Nb oxides to veins within the bounds of the enriched host rocks. Furthermore, fluorite, carbonates, and fluorocarbonates are abundant within the syenite, thus providing local complexing agents for the Be and REEs . Previously documented regional-scale thrust faulting and related deformation in the Pelly Mountains can explain the presence of dilational sigmoidal tension gashes as well as the presence of a metamorphic fluid. The source for excess silica required for the formation of quartz veins is not as easily explained since syenites are typically silica-undersaturated. However, three plausible sources for silica in the mineralizing fluid do exist and include (1) mobilized primary quartz i f the original composition of the syenite was locally quartz saturated, (2) mobilized quartz from syn-emplacement silicification, 113 (3) and/or mobilized 'secondary' quartz available during greenschist facies metamorphism. Unlike the previous two models, geochronological data of mid-Jurassic age obtained from fluorite supports this model of post-late-Triassic and pre-Cretaceous syn- to late tectonic mineralization. 114 | - 2 5 0 m 1 - Alkali exchange during emplacement of syenite (shaded area) 2 - Development of R E E - U - T h skarn within platform carbonates 3 - Primary igneous enrichment of rare elements 4 - Thrust faulting and regional tectonic compression 5 - Opening of dilational tension gashes with local gradients (arrows) for mass transfer 6 - Increased competancy of host rock at depth decreases potential size of open spaced gashes rzzi Beryl Allanite Siderite • Fluorite Wallrock fragment Quartz o Fe-Ti-Nb oxides Figure 56. Cartoon drawings of deposit model (a) and mineral ized tension gash (b) for beryl mineralization at the True Blue showing, southern Yukon Territory, C a n a d a . 115 C H A P T E R 9 E X P L O R A T I O N P A R A M E T E R S A N D T A R G E T S Introduction One of the goals of this project was to generate exploration parameters for beryl mineralization using the genetic model developed at the True Blue showing. A s stated in the introduction and described throughout the text, mineralization at the True Blue showing is distinct from other known beryl occurrences in Yukon. A s such, it expands the possibilities for individuals and exploration companies operating in Yukon. Parameters are defined and example targets in each of the Pelly Mountains, Yukon Territory, and the world are noted. Parameters A logical way of delineating pathfinders for a particular deposit model is to start from characteristics associated with only the mineralization itself and then move outwards in association to less direct and concrete associations. After these characteristics are described, one can then work backwards from a regional scale to a property scale. A t the True Blue occurrence, the beryl is hosted by quartz-carbonate-fluorite-allanite veins which cut an evolved Mississippian syenite. The fluid was locally derived, generated during regional metamorphism and routed into local low pressure dilatant tension gashes. N o one mineral was observed to occur exclusively alongside beryl; therefore, the only absolute mineral associated with mineralization is beryl itself. However, the events that gave rise to this mineralization do have broader traits that can be used to hone in on prospective areas, from distal (500 km 2 ) to medial (50 km 2 ) and proximal (5 km 2 ) ranges. The following three sections describe parameters divided into five subgroups relevant to the Pelly Mountains (geology, syenite mineralogy, vein mineralogy, geochemistry, physical processes), while Table 17 is a general summary of the parameters. 116 T A B L E 16. S U M M A R Y O F E X P L O R A T I O N P A R A M E T E R S A S D E S C R I B E D I N T E X T Dis ta l M e d i a l P rox imal G E O L O G Y -Syenite / evolved intrusion -Alkaline volcanic rocks ( P M V B ) -Thrust faulting -Syntectonic quartz veins -Regional metamorphic grade of greenschist to amphibolite facies -Roof zone of syenite (pendants, miarolitic, xenoliths) -Quartz veins in tension gashes -Fractures and/or shear zones -Preferred orientation of veins? S Y E N I T E M I N E R A L O G Y -Altered (albitized) syenite -Alka l i amphibole + pyroxene -Replacement perthite -Secondary R E E minerals (monazite, zircon, bastnaesite) V E I N M I N E R A L O G Y -Carbonates (ferroan?) -Other rare element minerals -Fluorite -Allanite -Nb-Ti-Fe oxides G E O C H E M I S T R Y -Be-REE-F-Th enrichment in stream silts -Be-REE-F-Th enrichment in soils -High N a / K in syenite P H Y S I C A L -Low amount fluorite, allanite, -High amount fluorite, allanite P R O C E S S E S and R E E oxide fragments in creeks and R E E oxide fragments in creeks Distal Because host rock is vital for providing the Be, it is the cornerstone o f target delineation. Identifying the location of Mississippian syenite bodies is facilitated through regional geologic maps and reports and through property-scale reports (e.g., Yukon Min ing Assessment Reports). Additional intrusive bodies not recognized at a regional scale may be found by focusing exploration to within the extents of associated coeval metavolcanics of the Pelly Mountain Volcanic Belt ( P M V B ) . Regional metamorphic grade throughout the Pelly Mountains is mid-greenschist to lower amphibolite, thus qualifying the whole of the P M V B as prospective. Proximity to traces of thrust faults is also important, as these structures record the location of increased shear stress, a likely requirement for producing the dilatant tension gashes that host mineralization. Syntectonic quartz veins and their geochemistry may be relevant, as they are evidence for crustal fluid flow and element mobilization. M e d i a l Once a prospective area with syenite outcrop is delineated conditions specific to the host should be examined. Roof zones are prospective as they w i l l be located close to contacts and therefore indicate a zone of highrheological differences. These zones can be revealed through roof pendants, miarolitic cavities, and sometimes by an increase in locally derived xenoliths. Proximity to shear zones is likely important, and areas with tension gashes or abundant quartz veins should be focused on. The mineralogy of the prospective host can also provide insight to mineralization. Thus far, mineralization at True Blue appears to be correlated with strong alteration and albitization characterized by flame perthite. The presence of alkali amphiboles and pyroxenes may also be relevant. Close attention to mineralogy of the veins is critical, as it w i l l indicate the geochemical characteristics of the mineralizing fluid. Carbonates are the most widespread group of minerals and can be used as outer limits to a mineralized zone. Silt samples from creeks may also give indications of rare-element enrichment. Although this method w i l l not indicate whether or not Be is hosted in beryl, it may reveal an enrichment in the catchment area. Creeks should also be examined for heavy mineral 118 fractions as allanite and Nb-Ti-Fe oxides have high specific gravities and are relatively durable. P rox ima l If a rare-element enriched syenite with tension gashes has been identified, mineralogical zoning within the veins is the most effective way to discover beryl. Phases of particular importance are those which contain complexing agents pertinent to Be (e.g., fluorite + [ferroan?]carbonate) and those which contain elements that use similar complexing agents (e.g., allanite, Nb-Ti-Fe oxides). Mineralogy of alteration within the host w i l l also help to delineate prospective zones by focussing on areas with altered zircon, secondary bastnaesite and monazite, and high N a / K values. Soil geochemistry may become useful at this scale, however like silt geochemistry it w i l l only indicate concentrations of elements and not the phases in which they reside. Abundance of heavy fractions in creeks of the aforementioned phases should also increase with increasing proximity to a mineralized zone. Targets wi thin Pelly Mountains Exploration records in Yukon M I N F I L E (Deklerk and Traynor 2004) contain references to small rare-earth element (REE)- and Th-enriched skarns and associated veining. These types o f R E E - T h occurrences sometimes note quartz-fluorite veins with similar characteristics to those seen at the True Blue showing. O f particular noteworthiness are the appearances of REE-bearing quartz-fluorite±carbonate veins cutting Mississippian igneous rocks at the Box (105F 021), M u m (105F 025), Nokluit (105F 080), Tier (105F 102), Coope (105G 092), and Whit (105G 105) Minfi le occurrences (Deklerk and Traynor 2004, and assessment reports referenced therein). A number of reports describing the geology and geochemistry of select V M S occurrences in the Pelly Mountains have also been published (e.g., Mor in 1977, Hunt 1999, Hunt 1997, Doherty 1997, Gibson et al. 1999). Theoretically, regions in proximity to all o f the V M S occurrences are prospective; however descriptions of the Mount Vermil l ion/Wolf prospect suggest that this region is promising. 119 Nokluit and K a y (Minfile 105F 080 and 105F 015) R E E - T h enriched veins containing fluorite cutting trachyte and syenite are reported at the Nokluit occurrence. Additionally, skarning is developed between Cambrian carbonates and Mississippian syenitic rocks and a swarm of quartz veins has been recognized ~1 km to the N W . A t the main showing, the vein assays report high concentrations of all rare earths, Nb , Th, U and Y , attaining up to 1.2% XREE2O3. Syenite in the area is also reported to have been highly metasomatized through the addition of silica and carbonate. The Kay occurrence is 1 k m east of Nokluit and is reported as a Mississippi Valley Type target where galena-tetrahedrite mineralization occurs in.quartz-barite-fluorite veins, fracture zones, and shear zones. These mineralized structures cut dominantly sedimentary formations, however volcanics are noted. This area 15 km east of the True Blue showing is prospective due to proven R E E enrichment, quartz-fluorite veins, 'fracture zones', and syenite outcrop. Additionally, the area is bounded to the west and east by normal faults and by a thrust fault to the south. Mount Vermill ion - Wol f (Minfile 105G 008) A report by Hunt (1999) describes the W o l f V M S deposit at Mt . Vermil l ion that is associated with the Pelly Mountain Volcanic Belt. This deposit is at the southeast end of the belt approximately 50 km SE of the True Blue occurrence. Layered rocks include felsic volcaniclastics, felsic to intermediate flows and dikes, and miscellaneous older and younger shallow water rocks such as carbonates and argillites. Volcanic rocks are altered by carbonation, chloritization, and the addition of Fe sulfides. Intrusive phases include syenite and syenite breccia. The syenite is described as locally pegmatitic (feldspar up to 4 cm long) and containing rare pyrite and fluorite. The syenite breccia comprises monolithic angular fragments of feldspar-quartz syenite with a matrix of smaller fragments and light to dark grey fine grained material. The syenite breccia is hypothesized to either record explosive igneous activity or represent a tectonic breccia. In support of the latter hypothesis is spatial proximity of the breccia to a mapped thrust fault. Most significantly, Hunt describes a diorite/andesite unit which is "strongly fractured with fracture/tension gashes" that are "filled with carbonate". 120 The Mount Vermil l ion area is prospective because of the syenite - metavolcanic contact that is spatially close to a thrust fault. Furthermore, a pegmatitic texture coupled with fluorite indicates that this is a volatile-rich and highly differentiated intrusion, suggesting enrichment in Be and REEs . The report of tension gashes in the area demonstrates that the appropriate deformational conditions are present for the opening of these features. Targets wi thin the Y u k o n Terr i tory M c E v o y Platform The M c E v o y Platform is situated northeast of Yukon-Tanana Terrane (YTT) , which is located north of the Tintina Fault. It has been postulated that the M c E v o y Platform is stratigraphically equivalent to the Cassiar Platform (Gordey and Makepeace 2003). This area also contains reports of alkaline volcanics and V M S showings that are significantly different that those of the Y T T (Fonseca 2001). Although no intrusive equivalents have been recognized for the felsic alkaline volcanics from here, the possibility of a syenite subvolcanic equivalent, and therefore Be source, is intriguing. This area has also undergone extensive compressional deformation and associated metamorphism (Fonseca 2001), thus permitting the possibility of regional metamorphic fluids. Selwyn Basin Another package of rocks that contains alkaline volcanics and that has undergone similar compressional tectonic events is those of the Selwyn Basin (Gordey and Makepeace 2003, Holbek et al. 2000). The Marg V M S deposit (Minfile 106D 001) is the most noteworthy and is thought to reside at the boundary between off and on-shelf stratigraphy (Holbek et al. 2000). These rocks have undergone upper greenschist facies metamorphism and three main thrust faults are present in the vicinity of the deposit. The parental magma to these volcanics and perhaps the volcanics themselves may be enriched in exotic elements such as Be and the R E E . Although these rocks are likely not as prospective as those found within the Cassiar Platform they still deserve notice. 121 Additionally, black shales of the Selwyn Basin have also been noted to be reminiscent of the shales that host the Colombian emerald deposits (Lewis et al. 2003, Walton 2004). Targets outside the Y u k o n Terr i tory Southeast European Alps Because regional thrusting is requisite for this model of beryl formation, it can be postulated that many thrust belts are prospective. However, additional constraints are that the thrust panels must contain pre-metamorphic Be-enriched rock (i.e. syenite) that is T h e o l o g i c a l l y different from its host rocks, thereby increasing the probability for open space tension gash formation and gem beryl precipitation. Therefore, portions along the whole of Western Canada (BRITISH C O L U M B I A + Yukon) and northwestern U S (i.e. Idaho Thrust Belt) are theoretically prospective. Many thrust belts exist globally and a good example (and one which reports beryl) is the southeast European Alps , which has undergone Tertiary Alpine metamorphism. The Alps host the historical discoveries of "Alpine Clefts", provide fantastic situations to study brittle and ductile deformation of granitoids, and also offer environments in which mid-crustal fluid movement and mobilization of trace elements can be readily studied (e.g., Guermani and Pennacchioni 1996, Henry et al. 1996, Rolland et al. 2003). Guermani and Pennacchioni (1996) described in detail two outcrops (-100 each) of the Mont Blanc Massif with many brittle and ductile features. O f particular note was their confirmation that brittlely emplaced planar to sigmoidal veins occurred late in Alpine orogenesis and that their vein mass was heavily influenced by the host. This host granite appears undeformed at outcrop scale; however petrographic examinations revealed extensive internal deformation and stress induced flame perthites. Henry et al. (1996) describe an interconnected syn-metamorphic vein network that is dominated by local fluids but confined to discrete tectonic units. Rolland et al. (2003) examined a larger system comprising mid crustal shear zones. They determined that trace elements within vein-shear hosted rare-element minerals originated from the alteration of primary minerals with essential trace element components (i.e. monazite, bastnaesite, e t c . ) . 122 Their study also focused on the Mont Blanc massif and report that hydrothermal veins formed synchronously with the shear zones. These studies and many others (e.g., Brugger et al. 1998, Eichhorn et al. 1999, Brugger and Giere 2000, Cesare et al. 2001, Widmer and Thompson 2001) provide evidence that this region has undergone similar tectonic events and metamorphic grades, although the geochemical signature of the studied host rocks is not conducive to beryl mineralization. However, rocks of the Tauern Window of the Eastern Alps endured similar tectonic events and appear to have more favorable geology and geochemistry. There, the F e - M n deposits of the V a l Ferrera and the W deposits of the Ferbertal regions both show rare-element enrichment and late Alpine orogenic remobilization. In fact, the V a l Ferrara region contains several reports of a late forming beryl within hematite-quartz-dolomite-powellite veins that cut dolomite breccia, as briefly noted in Chapter 1. Brugger et al. (1998) and Brugger and Giere (2000) report these discordant veins as having a geochemical signature similar to their hosts and postulate that they were emplaced late during Alpine deformation (likely pre- to syn-D3). The dolomite breccias are also said to be more competent than the surrounding rocks and responded in a brittle manner to deformation, thus producing open spaces for crystals to grow freely. The authors also suggest that the components in the veins were likely locally mobilized from the wall rocks and precipitated in the veins. It is interesting to note that the Habachtal emerald deposit is nearby and its origin is tied to the shear/blackwall contact between a Be-enriched felsic gneiss and Cr-enriched metaperidotite. Thus the Tauern Window has direct and indirect evidence of beryl mineralization with similar conditions to beryl mineralization at True Blue. S u m m a r y Like many gemstone models, regional and local geology are currently the best ways to initiate exploration for additional occurrences. Because exploration for gemstones is less common than for base and precious metals, information pertaining to gems is not typically recorded. Consequently, extracting pertinent information from reports focused on other deposit types and geologic phenomena is an important ski l l . For example, within the Pelly Mountains historical information recorded in government 123. mining and exploration records focus primarily on V M S occurrences and therefore do not usually contain much information on outcropping syenite. Often, the presence of syenite w i l l be stated, however no textural, chemical or structural information w i l l be given. In this case, sieving through the dril l logs for mention of indicators such as quartz veins, fluorite or particularly heavy alteration can be worthwhile. Ultimately, once a high priority target has been identified the most effective exploration technique is ground prospecting and careful, methodical observation. 124 C H A P T E R 10 C O N C L U S I O N Conclusions A descriptive and genetic model has been developed for beryl mineralization for the True Blue showing and an exploration model has been built from the interpretations. The most likely scenario for beryl mineralization at True Blue is that it occurred during the mid-Jurassic after greenschist facies regional metamorphism but synchronous with or shortly after cessation of regional thrusting. The veins f i l l sigmoidal tension gashes in altered syenite, and several lines of evidence suggest that Be and other exotic elements were locally sourced from the syenite and mobilized by fluoride and carbonate complexes. Future research This project has satisfactorily resolved a number of issues regarding the origin of mineralizing fluids at the True Blue gem beryl showing. However, as with most research projects with every answer come several more avenues for continued investigation. Three areas (of many) that warrant further investigation are briefly discussed. (1) This deposit-scale project and resulting syn-tectonic mineralization model has generated spatial and temporal implications for gem beryl exploration models concerning not only southern Yukon but also all along the western Cordillera. Furthermore, insight on the regional geology of the Pelly Mountains can be obtained and, by extension, insight to regional geology of the western Cordillera. For instance, the observations at True Blue help characterize a regional metamorphic event and geochronological data obtained in this study allows a timestamp to be placed on this spatial-temporal region that is largely understudied. These implications deserve additional time and effort. (2) The unusually dark blue colour and anomalously high Fe content of beryl from the True Blue showing has allowed in-depth investigations into the complex crystal chemistry of this gem mineral. 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Acta 57', 1079-1091. 142 A P P E N D I X 1: A N A L Y T I C A L P R O C E D U R E S Powder-diffraction data were collected at U B C with a Siemens D5000 diffractometer equipped with a diffracted-beam graphite monochromator and a C u X-ray tube operated at 40 k V and 40 m A . For mineral identification data were collected from 3 to 70° 20 with a scanning step-size of 0.04° 20 and step time of 0.8 seconds. Whole-rock analyses o f the syenite were done by A L S Chemex Limited of North Vancouver. Major elements were analyzed by X R F (method M E - X R F 0 6 ) and trace elements not listed below were analyzed using lithium borate fusion and I C P M S (method M E - M S 8 1 ) . Beryllium, L i , and Sc were analyzed via I C P - M S after a four-acid digestion (method M E - M S 6 1 ) and B was analyzed via fusion and I C P - A E S (method B-ICP82). Chlorine was analyzed by neutron activation analysis and specific ion potentiometry (methods C L - N A A 0 6 , C L - E L E 8 1 A ) . Fluorine was also analyzed be specific ion potentiometry (method F-ELE81) . Carbon and CO2 were analyzed by coulometer. Ferrous iron was analyzed by HC1-HF acid digestion and titration methods ( F e - V O L 0 5 ) . Primary H 2 0 was determined through infrared techniques (OA-IR06) and moisture content was determined by gravimetric methods after sample drying (method O A -GRA10) The Philips X L 3 0 scanning electron microscope (SEM) at the University of British Columbia, which is equipped with an energy-dispersion X-ray spectrometer (EDS), was used for preliminary examination of mineral mounts and polished thin sections. A fully automated Cameca SX-50 microprobe was used for analysis (wavelength-dispersion mode). Operating conditions were as follows: voltage 15 k V , current 10 n A (phyllosilicates, carbonates) and 20 n A (other phases), beam diameter 5 u.m (allanite, feldspar, oxides, zircon), 10 urn (phyllosilicates), 20 urn (carbonates), and 25 um (beryl). Peak count time for standards was 40 seconds and background count time was 10 seconds. Standards consisted of natural minerals, synthetic minerals and glasses, and elemental metals. Data reduction was done according to the " P A P " <))(pZ) method of 143 Pouchou and Pichoir (1991). Information pertaining to standards and X-ray lines can be found in Appendix 4. Microthermometric analyses of the fluid inclusions were conducted at the University of Waterloo using a Linkham T H M S G 6 0 0 heating-freezing stage on polished offcuts from thin sections and polished rock chips. The stage was calibrated using synthetic fluid inclusion standards and temperatures below zero are accurate to within ±0.1 °C and at high temperature to within ±1°C. In situ stable isotope data for the beryl crystals were collected at C R P G / C N R S in Vandoeuvre (France) according to the procedure of Giuliani et al. (1997). The crystals were first heated to 500 °C to ensure decrepetation of the fluid inclusions. Oxygen isotopic ratios ( l s O / 1 6 0 ) for the beryl crystals were determined using a Cameca 1270 ion probe with a C s + primary beam and electron bombardment. Analyses of the 1 6 0 and 1 8 0 secondary ions were done in multicollection mode, at a mass resolution of 4500. The instrumental mass fractionation was calibrated with a set of three standards of different 18 16 compositions. The 0 / O ratios were determined with a precision of 0.4%O(1CT). Oxygen isotope data for quartz, beryl, siderite, and calcite from the quartz veins associated with the beryl mineralization were obtained at Queen's Facility for Isotope Research (QFIR) located at Queen's University, Ontario. Quartz and beryl were soaked in 4 N HC1 for 10 minutes sonicated 3 times in distilled water for 8 minutes. A bromine pentafluoride extraction line was used for oxygen in silicates and the isotopes were measured with a Finnigan Mat 252 Mass spectrometer. Carbonate samples were dissolved in H3PO4 and 1 8 0 / 1 6 0 ratios were measured from released CO2. Calcite was reacted for 4 hours at 25 °C while siderite was reacted for 48 hours at 50 °C. Carbonate analyses were also measured with a Finnigan Mat 252 Mass spectrometer. Six samples of fluorite with colours ranging from clear to dark purple were chosen • for Sm-Nd isotopic analysis at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia ( U B C ) . Optically clean samples were • r . 144 hand picked and crushed with an agate mortar and pestle. The samples underwent two dissolution steps in Teflon vessels at 140 °C for 48 hours each. The first dissolution was done with 14 N H N 0 3 and the second with 6 N HCI and 14 N H N 0 3 . After digestion, an aliquot equivalent to 30-100 mg sample was taken for trace element analysis (the remainder was used for isotope analysis) and taken to dryness. The samples were subsequently diluted 1000x (gravimetrically) using a stock solution of 1% HNO3 with 1 ppb In in 125 m L H D P E sample bottles. Indium is used as an internal standard to monitor instrumental and sensitivity drift. The instrument used for trace element analysis was a Thermo Finnigan Element2, a double focusing (i.e., high-resolution) Inductively Coupled Plasma-Mass Spectrometer (HR- ICP-MS) . The Element2 sample introduction system consists o f a self-aspirating, low-uptake rate (100 )nL/min) Microflow P F A nebulizer (Elemental Scientific, Omaha), coupled to a cooled, Scott-type glass spray chamber. Aluminum sample and skimmer cones from R A Chilton ( U K ) were used. Pb and U were measured in low resolution mode (LR) in order to take advantage of higher instrument sensitivity. The R E E s were measured in high resolution mode (HR) in order to avoid any potential interference on the H R E E s arising from LREE-oxides . The REEs , U and Pb were quantified using external calibration. Five multi element standards were prepared from High-Purity (USA) single element commercial standards prior to each analysis session. Procedural blanks were in the part-per-trillion (ppt) range for each element considered, which is negligible with respect to the abundance of the elements and high dilution factors. The instrument was rinsed between sample acquisitions with 1% HNO3 for 2 minutes. Background and low abundance standard solutions were analyzed after every five samples in order to monitor memory effects and instrumental mass drift, both of which were found to be negligible. R E E s were obtained through cation exchange geochemistry following the steps described in Weis et al. (2005). Sm and N d were then obtained through a second round of column chemistry, again following the steps described in Weis et al. (2005). After column chemistry, Sm and N d isotope ratio measurements were carried out with a Thermo Finnigan Triton-TI T I M S instrument in static mode with relay matrix rotation on a double Re-Ta filament. 135 cycles were usually completed (9 blocks of 15) 145 for each analysis to allow full rotation of the virtual amplifier. Sample N d isotopic compositions were corrected using 1 4 6 N d / 1 4 4 N d = 0.7219. During the course of analyses, 10 analyses of the L a Jolla N d standard were made, with a mean value of 1 4 3 N d / 1 4 4 N d = 0.511853 ± 0.000011 (2a). , 4 7 S m / 1 4 4 N d ratio errors are approximately -1 .5%, or -0.006. Isoplot 3.0 was used for age regression and error calculations (Ludwig 2003) of the Sm-N d data with ?i 1 4 7 Sm = 6.54 x 1 0 1 2 year"'. Handpicked allanite from four rock samples was chosen for initial consideration factors concerning suitability included apparent alteration, quality of crystallization (i.e. euhedral crystals) and volume of material available. Mineral separates were made by hand picking and crystal fragments were investigated under a binocular microscope. Two sample populations (samples 37 and 57) were then chosen based on good translucency and the colour of the crystal in transmitted light. Portions of these two samples were then mounted on a stub for S E M imaging to check for alteration. Long axes were oriented both parallel and perpendicular to the imaging stage. Populations suitable for U-Pb geochronology were abraded to remove any alteration on the outside of the crystals prior to wet chemistry. Geochemical separations and mass spectrometry were done at the Pacific Center for Isotopic and Geochemical Research in the Department of Earth and Ocean Sciences, University of British Columbia. Allanite was weighed and dissolved in concentrated H F and HNO3 in the presence of a mixed " U - Pb tracer in 3 m L screwtop P F A beakers at 120 °C for a minimum of 3 days. This solution was then evaporated to dryness and salts were taken up in 6.2 M HCI . This solution was dried and salts were taken up in 3.1 M HCI . Separation and purification of Pb and U employed ion exchange column techniques modified slightly from those described by Parrish et al. (1987). Pb and U for each fraction were eluted sequentially into a P F A beaker and loaded together on a single Re filament using a phosphoric acid-silica gel emitter. Isotopic ratios were measured using a modified single collector V G - 5 4 R thermal ionization mass spectrometer equipped with a Daly photomultiplier. Measurements were done in peak-switching mode with a Daly detector. U and Pb analytical blanks were in the range of 1 pg and 2-4 pg, respectively, during the course of this study. U fractionation was determined directly on 146 individual runs using the 2 i i ' ' Z i i \ ] tracer, and Pb isotopic ratios were corrected for a fractionation of 0.32%/amu based on replicate analyses of the NBS-982 Pb standard. A l l analytical errors were numerically propagated through the entire age calculation using the technique of Roddick (1987). A l l age errors are quoted at 2a (Ma) level and'errors for isotopic ratios are quoted at l a (%). 147 


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