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.
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.
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.
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.
Fraser Lakes Geology
Modified from Ko, 1971

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