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McKechnie et al. GAC-MAC 2013 Presentation

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Oral Presentation at the GAC-MAC 2013 Conference in Winnipeg, MB.

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McKechnie et al. GAC-MAC 2013 Presentation

  1. 1. Petrogenetic model for U-Th-REE mineralized granitic pegmatites in the high-grade metamorphic rocks of the Wollaston Domain, Saskatchewan: Evidence from Fraser Lakes Zone B McKechnie, Christine L., Annesley, Irvine R. , and Ansdell, Kevin M. GAC-MAC 2013 Winnipeg, MB
  2. 2. Outline  Geological Setting of Fraser Lakes Zone B  Metamorphism + Migmatization  Model for the Fraser Lakes Zone B U-Th- REE deposit  Structural and Geochemical Controls  Similarities to other pegmatite-hosted U deposits  Implications for U exploration in Saskatchewan
  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)  Paleoproterozoic Wollaston Group metasedimentary rocks  Hudsonian granites, amphibolites, migmatites, leucogranites, and granitic pegmatites  Study area shown in red box McKechnie et al. in press
  4. 4. Fraser Lakes Geology • NE-SW regional fabric • 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. Granitic pegmatites and leucogranites • Granitic pegmatites and leucogranites with variable amounts of quartz, feldspar, biotite, and other minerals • Overall coarse grained to pegmatitic • Variable width (cm to dm scale) • Complexly zoned (igneous AFC processes) • Multiple generations of granitic pegmatites • 1850-1780 Ma U-Pb chemical ages (CHIME) for magmatic uraninite
  6. 6. Metamorphic P-T-t path  (1) Prograde metamorphism above the second sillimanite isograd  Garnet cores w/ biotite and sillimanite inclusions (no muscovite or cordierite)  (2) Peak thermal metamorphism  Garnet – biotite – sillimanite – cordierite – K-feldspar – melt assemblages and abundant leucosome  (3) Retrograde metamorphism during decompression  Spinel – sillimanite and garnet – quartz – biotite symplectites McKechnie et al. 2012
  7. 7. Granitic Pegmatites / Leucogranites – Partial melting at depth vs. in-situ?  Migmatites in close association with the radioactive intrusives  Leucosomes tend to be boudinaged, but also form small pegmatitic veins ► Crystallized melt occasionally forms thin rims around minerals, and locally larger blobs ► Biotite frequently shows degradation due to partial melting
  8. 8. Model for Fraser Lakes Zone B  (1) Melting of the source rocks 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  (3) Retrograde metamorphism, and associated alteration due to fluids moving through the rocks McKechnie et al. 2012
  9. 9. Structural controls  Two main structural controls at Fraser Lakes Zone B  (1) Archean- Wolllaston Group contact  Sheared contact  Rheological contrasts  (2) Antiformal fold noses and other dilational zones Mercadier et al. (in press) McKechnie et al. 2012
  10. 10. Group A vs. Group B Granitic Pegmatites/Leucogranites 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 Group B Intrusives  Monazite-rich; i.e. Th + LREE-rich  Composition suggestive of more “restitic” sources  Monazites forms large clusters with biotite  Zircon contain inherited cores  Central part of the fold nose
  11. 11. Geochemical/Mineralogical Controls ► Differences in source rocks? ► Group A – little to no monazite, uraninite-bearing (U-rich source needed) ► Group B - Inherited monazite – from source as no large monazite in surrounding host rocks, ages are older than expected (2.1 to 2.2 Ga) ► Degree of melting ► Group A is U-rich, U would be concentrated in earlier melts ► Group B likely from melting of an already melted source (restite)
  12. 12. Geochemical/Mineralogical Controls cont’d ► Archean-Wollaston Group contact ► Redox control ► Magnetite in pegmatites intrusive into Archean gneisses only ► More U concentrated at margins of pegmatites that are in contact with reduced lithologies (i.e. graphitic pelitic gneisses) ► 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
  13. 13. Comparison with other pegmatite/leucogranite-hosted deposits  Primary magmatic mineralization with variable secondary overprint  Derived from partial melting of metasedimentary gneisses at depth during peak thermal metamorphism  Mineralized pegmatites/granitoids concentrated in areas of higher metamorphic grade  Granitic to pegmatitic textures and “granitic” (sensu lato) compositions  Differences in composition and uranium concentration are likely due to different sources and amounts of transport and assimilation-fractional crystallization  Melts concentrated preferentially in antiformal fold noses and along shear zones as sheeted bodies, like at the Rössing and Husab (formerly Rössing South) deposits McKechnie et al. 2012 (Modified from Ray, 1979) Extract Resources, 2009
  14. 14. Implications for granitoid-hosted U mineralization in Saskatchewan After Parslow and Thomas, 1982  NI 43-101 compliant initial resource estimate completed at FLZB (see the next talk by Annesley et al.) indicated a small resource (~ 6.9 million lbs U3O8 @ 0.030 %); still open at depth/along strike  Several other showings in SK; most with limited work  Work has recently been done to the SW of Fraser Lakes Zone B by Eagle Plains Resources  Ignored due to proximity to the Athabasca Basin and lower grades relative to unconformity-type deposits  Potential exists for more significant finds in the rest of the Wollaston Domain and adjacent Mudjatik Domain
  15. 15. Conclusions  Basement-hosted, magmatic U and Th mineralization (+/- REE mineralization)  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, and others  Magmatic U mineralization may represent a new type of economic uranium deposit in northern Saskatchewan
  16. 16. Questions?

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