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

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

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

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

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

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) Retrogrademetamorphism during decompression

Spinel – sillimanite and garnet – quartz –biotite symplectites

McKechnie et al. 2012

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

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

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

Group A vs. Group B Granitic

Pegmatites/LeucogranitesGroup 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

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)

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

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

Extract Resources, 2009McKechnie et al. 2012 (Modified from Ray, 1979)

After Parslow and Thomas, 1982

Implications for granitoid-hosted U

mineralization in Saskatchewan 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

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

Questions?