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U-TH-REE-MINERALIZED GRANITIC 
PEGMATITES FROM FRASER LAKES ZONE 
B: FERTILE CRUSTAL MELTS AND 
POTENTIAL U PROTORE? 
McKe...
Outline 
• Geological Setting of Fraser Lakes Zone B 
• Pegmatite geology 
• Model for the Fraser Lakes Zone B U-Th- 
REE ...
Regional Geology 
 Hearne Province 
 Deformed and 
metamorphosed during the 
Paleoproterozoic (ca. 1.9-1.8 
Ga) Trans-Hu...
Fraser Lakes Geology 
• NE-SW regional fabric 
• Two granite- 
/pegmatite-hosted U-Th- 
REE showings, 
Zones A and B, in t...
Fraser Lakes Geology 
Modified from Ko, 1971
Granitic pegmatites and leucogranites 
• Granitic pegmatites and leucogranites 
w/ variable amounts of quartz, 
feldspars,...
Mineralogy 
Highly Variable! 
U-Th-REE Minerals 
• Uraninite (Urn) 
• Thorite +/- U (Th) 
• Monazite (Mz) 
• Zircon (Zrn) ...
Group A vs. Group B Intrusives 
Group A Intrusives 
• Contain abundant uraninite, thorite, and 
zircon (inherited cores) a...
Granitic Pegmatites / Leucogranites – 
Possible relationship to partial melts 
 Migmatites in close association (i.e. 
ho...
Metamorphic Mineral Assemblages in 
host migmatitic pelitic gneisses 
• Garnet 
• Biotite 
• Cordierite 
• Sillimanite 
• ...
Model for Fraser Lakes Zone B 
McKechnie et al. 2012 b 
• Later retrograde 
metamorphism, 
and associated 
alteration due ...
Structural 
controls 
•Two main 
structural controls 
at Fraser Lakes 
Zone B: 
• (1) Archean- 
Wolllaston Group 
contact ...
Host rock Controls 
• Pegmatites intruding the 
Archean gneisses contain 
magnetite and more K-spar 
• No magnetite (only ...
Geochemical/Mineralogical Controls 
Group A vs. Group B 
• Differences in source rocks and degree of melting? 
• Group A -...
Comparison with other 
pegmatite/leucogranite-hosted U deposits 
• Primary magmatic U mineralization with variable seconda...
U protore? 
• Been proposed (Annesley et al. 2000, 
Mercadier et al. 2013, and others) that 
radioactive pegmatites may be...
Conclusions 
• Basement-hosted, magmatic U and Th mineralization (+/- REE 
mineralization) 
• Abyssal-class pegmatites (us...
Questions?
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McKechnie and Annesley GSA 2014 Presentation

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Oral Presentation given by Christine McKechnie at the GSA 2014 Conference in Vancouver, B.C.

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McKechnie and Annesley GSA 2014 Presentation

  1. 1. U-TH-REE-MINERALIZED GRANITIC PEGMATITES FROM FRASER LAKES ZONE B: FERTILE CRUSTAL MELTS AND POTENTIAL U PROTORE? McKechnie, Christine L. and Annesley, Irvine R. GSA 2014 Vancouver, B.C., Canada
  2. 2. Outline • Geological Setting of Fraser Lakes Zone B • Pegmatite geology • Model for the Fraser Lakes Zone B U-Th- REE deposit • Structural and Geochemical Controls • Comparison w/ other pegmatite-hosted U deposits • U protore?
  3. 3. Regional Geology  Hearne Province  Deformed and metamorphosed during the Paleoproterozoic (ca. 1.9-1.8 Ga) Trans-Hudson Orogeny (THO)  In the Eastern Wollaston Domain, which consists of: Archean orthogneisses (mostly granitic) PaleoproterozoicWollaston Group metasedimentary rocks Hudsonian granites, amphibolites, migmatites, leucogranites, and granitic pegmatites  Study area shown in red box McKechnie et al. 2012 a, b, 2013
  4. 4. Fraser Lakes Geology • NE-SW regional fabric • Two granite- /pegmatite-hosted U-Th- REE showings, Zones A and B, in the vicinity of Fraser Lakes • Zone A is in a NNE-plunging synformal and Zone B is in an NNE-plunging antiformal fold nose • 5 km section of a complexly folded electromagnetic (EM) conductor (i.e. graphitic pelitic gneisses) is adjacent to Zones A and B After Ray, 1979 Fraser Lakes Zone B Fraser Lakes Zone A
  5. 5. Fraser Lakes Geology Modified from Ko, 1971
  6. 6. Granitic pegmatites and leucogranites • Granitic pegmatites and leucogranites w/ variable amounts of quartz, feldspars, biotite, and other minerals • Inequigranular grain size distribution; overall very coarse grained (pegmatitic) • Graphic intergrowths are common • Variable width (cm to dm scale) • Complexly zoned (igneous AFC processes), zoning is variable between pegmatites • Multiple generations of pegmatites, syn-tectonic (subcordant to gneissosity, often radioactive) and post-tectonic (discordant, non-mineralized) • Sharp contacts w/ host rocks
  7. 7. Mineralogy Highly Variable! U-Th-REE Minerals • Uraninite (Urn) • Thorite +/- U (Th) • Monazite (Mz) • Zircon (Zrn) • Allanite (Aln) • Xenotime (Xen) Primary Minerals • Quartz (Qtz) • Feldspar (Fsp) • Biotite (Bt) • Magnetite (Mgt) • Ilmenite (Ilm) • Pyrite (Py) • Fluorite (Fl) • Sphalerite • Molybdenite • Apatite (Ap) • Titanite • Rutile • Garnet • Chalcopyrite • Pyrrhotite • Graphite • Nb-oxide * Magmatic and/or peritectic minerals
  8. 8. Group A vs. Group B Intrusives Group A Intrusives • Contain abundant uraninite, thorite, and zircon (inherited cores) and minor allanite • Less biotite and other “restite” minerals like Grt, Crd, etc. • Intrude the western part of the antiformal fold nose • U-Th-Pb chemical ages (uraninite) of 1.85-1.80 Ga • Tend to be more Si-enriched (McKechnie et al. 2013) • Abyssal-U (AB–U) subclass Group B Intrusives • Monazite-rich; i.e. Th + LREE-rich, w/ zircon (inherited cores), thorite, xenotime, allanite • More “restite” minerals like Grt, Crd, Bt, etc. • Monazite forms large clusters with biotite, is often partially resorbed • Intruded the central part of the fold nose • U-Th-Pb chemical ages (monazite) of 2.1 to 2.2 Ga, but field relationships suggest a similar age to the Group A intrusives • LREE (AB–LREE) subclass
  9. 9. Granitic Pegmatites / Leucogranites – Possible relationship to partial melts  Migmatites in close association (i.e. hosting) the radioactive intrusives  Leucosomes tend to be boudinaged, but also form small pegmatitic veins  Crystallized melt? in thin section  Biotite frequently shows degradation due to partial melting ► No nearby granite of similar age, yet field relationships suggest that the migmatites are possibly similar in age to the pegmatites.
  10. 10. Metamorphic Mineral Assemblages in host migmatitic pelitic gneisses • Garnet • Biotite • Cordierite • Sillimanite • Spinel • Quartz • Plagioclase • K-feldspar • Rutile • Myrmekite • NO prograde muscovite Upper amphibolite to granulite facies peak thermal metamorphism (750 to 780°C, 6 to 8 kbar) @ ~1.8 Ga
  11. 11. Model for Fraser Lakes Zone B McKechnie et al. 2012 b • Later retrograde metamorphism, and associated alteration due to fluids moving through the rocks • (1) Melting of source rocks at depth containing abundant U-Th- REEs via Bt-dehydration reactions [Bt + Qtz + (Sil)  Grt + Crd + (Kfs + L)] • (2) Migration along melt pathways to where it was crystallized in the middle crust
  12. 12. Structural controls •Two main structural controls at Fraser Lakes Zone B: • (1) Archean- Wolllaston Group contact • Sheared contact • Rheological contrasts • (2) Antiformal fold nose Mercadier et al. 2013 McKechnie et al. 2012 b
  13. 13. Host rock Controls • Pegmatites intruding the Archean gneisses contain magnetite and more K-spar • No magnetite (only ilmenite), higher MgO/TiO2 ratios in pegmatites intruding the Wollaston Group metasediments • More U concentrated at margins of pegmatites that are in contact with reduced lithologies (i.e. graphitic pelitic gneisses) • Similar to the redox control proposed for the Orrefjell Pegmatite-hosted Uranium Project in northern Norway (Mikkel Vognsen, 2010 PDAC)
  14. 14. Geochemical/Mineralogical Controls Group A vs. Group B • Differences in source rocks and degree of melting? • Group A - little to no monazite, uraninite-bearing (U-rich source needed, U would have been concentrated in earlier melts) • Group B - contain inherited monazite (most likely from the melt source based on size and age), no uraninite (so U-depleted source?), more “restite” minerals (i.e. melt generated from a more residual source) • Amount of melt transport and AFC processes • Group A – more restite unmixing due to farther from source rocks, and more evolved composition • Group B – more restite minerals, less restite unmixing
  15. 15. Comparison with other pegmatite/leucogranite-hosted U deposits • Primary magmatic U mineralization with variable secondary overprint • Derived from partial melting of metasedimentary gneisses at depth during peak thermal metamorphism; no relationship to any large granitic intrusions • Granitic to pegmatitic textures and “granitic” (sensu lato) compositions • Differences in composition and U concentration are likely due to different sources, amount of transport and assimilation-fractional crystallization, and host rock composition • Melts concentrated preferentially in antiformal fold noses and along shear zones as sheeted bodies, like at the Rössing and Husab (formerly Rössing South) deposits in Namibia • Other similar pegmatites/granites are found in the Svecofennian Orogen of Norway and the Grenville Province; several other occurrences have also been found in Saskatchewan McKechnie et al. 2012b (Modified from Ray, 1979) Extract Resources, 2009
  16. 16. U protore? • Been proposed (Annesley et al. 2000, Mercadier et al. 2013, and others) that radioactive pegmatites may be a major source of U for unconformity-type U deposits in the nearby Athabasca Basin • Chlorite, clay (including illite), and hematite alteration found in drill core • Erosion at FLZB was to an estimated depth of 150-200 m below the Athabasca/ basement unconformity • Brittle faulting cross-cuts the mineralized zone • Conduit for fluid and heat flow? • Uranium (and other metals) remobilized along fractures away from primary magmatic uraninite • Alteration of monazite may have also led to uranium remobilization • No basement-hosted, unconformity-related mineralization has yet to be intersected during drilling activities in the area (but it may exist) McKechnie et al. 2013
  17. 17. Conclusions • Basement-hosted, magmatic U and Th mineralization (+/- REE mineralization) • Abyssal-class pegmatites (using Černý & Ercit 2005 classification scheme) • Hosted by Hudsonian granitic pegmatites and leucogranites intruding at/near the highly deformed contact between Wollaston Group metasediments and Archean orthogneisses • Formed by partial melting of metasedimentary rocks in the middle to lower crust followed by transport and assimilation-fractional crystallization • Strong structural control on the mineralization by the unconformity between the Wollaston Group and Archean gneisses and the regional antiformal fold nose • Similarities to Rössing and Husab (Rössing South) granitoid-hosted U deposits in Namibia, Orrefjell Uranium Project in Norway, and others • Magmatic U mineralization may represent a new type of economic uranium deposit in northern Saskatchewan or protore for unconformity-releated U deposits
  18. 18. Questions?

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