This document describes a petrogenetic model for uranium mineralization hosted in granitic pegmatites in the Wollaston Domain of Saskatchewan. Partial melting of metasedimentary rocks during peak metamorphism generated uranium-rich melts. These melts migrated along structural pathways and crystallized in the middle crust, forming the granitic pegmatites. The mineralization shows structural control along the contact between Archean gneisses and younger metasediments, as well as within antiformal fold noses. Differences in uranium concentration between pegmatite groups relate to source composition and degree of melting/fractional crystallization. The deposit shares similarities with other granitoid-hosted uranium deposits and

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Questions?