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McKechnie and Annesley GSA 2014 Presentation
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
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
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
Refolded fold structure – polyphase folding – defined by lithological units and EM conductor
Superimposed ductile-brittle (E-W) and brittle structures (NNE and NNW)
Hint that pink is Archean, grey, yellow and blue metasediments, intrusives (pink-pegmatites)