gac-mac 2011 - austman et al - origin, geology, and composition of fraser lakes zone b u-th-ree...

The geological setting, composition, and origin of the Fraser Lakes Zone B granitic

pegmatite-hosted U-Th-REE mineralization, Wollaston Domain, northern Saskatchewan, Canada

Austman, Christine L. 1, 3

Annesley, Irvine R. 1, 2, and Ansdell, Kevin M.1

1 Department of Geological Sciences, University of Saskatchewan2 JNR Resources Inc., Saskatoon, SK3 CanAlaska Uranium Ltd., Saskatoon, SK

GAC-MAC 2011May 2011

Outline

• Purpose• Regional Geological Setting• Fraser Lakes Geology• Granitic Pegmatite Mineralogy and

Geochemistry• Metamorphism and Migmatization• Model for Pegmatite Generation

Purpose of this study

Describe the granitic pegmatite-hosted U-Th-REE mineralization at Fraser Lakes Zone B

Develop a metallogenetic model for pegmatite-hosted U-Th-REE mineralization in the Fraser Lakes area

Build upon previous work done on pegmatite-hosted uranium mineralization in northern Saskatchewan

Regional Geology The Wollaston

Domain consists of: Archean

orthogneisses (predominantly granitic gneisses)

Paleoproterozoic Wollaston Supergroup metasedimentary and metavolcanic rocks

Hudsonian granites, amphibolites, leucogranites, migmatites, and granitic pegmatites

Complexly deformed during the 1.8 Ga Trans-Hudson Orogen

Fraser Lakes Geology

• NE-SW regional fabric• Two mineralized

zones: A and B• 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 L

akes Gra

nite In

lier

(Arc

hean)

Johnson River G

ranite

Inlier (

Archean)

Wolla

ston G

roup m

etas

edim

enta

ry

gneiss

esWollaston G

roup metasedim

entary

gneisses

Fraser Lakes Zone B

Fraser Lakes Zone ANeedle Falls

Shear Z

one

Peter Lake D

omain and

Wath

aman Bath

olith

Fraser Lakes Zone B

• The surface expression of the EM conductor adjacent to Fraser Lakes Zone B is a swampy, low-lying area

• Zone B mineralization outcrops at surface on the eastern edge of the swamp

• Fold nose is visible from the air

Fraser Lakes Zone B geology

Modified from Ko, 1971

WYL-08-526

WYL-08-524WYL-08-525

Granitic pegmatites

• Granitic pegmatites with variable amounts of quartz, feldspar, biotite, and other minerals

• Overall coarse grained to pegmatitic

• Variable size (cm to several m scale)

• Complexly zoned (igneous AFC processes)

• Multiple generations of granitic pegmatites (late-tectonicto post-tectonic)

• 1810-1790 Ma uraninite U-Pb chemical ages in mineralized pegmatites

Mineralogy

• Quartz (Qtz)• Feldspar (Fsp)• Biotite (Bt)• Magnetite (Mgt)• Ilmenite (Ilm)• Pyrite (Py)• Fluorite (Fl)• Sphalerite

Highly Variable!

• Uraninite (Urn)• Uranothorite

(Uth)• Monazite (Mz)

• Zircon (Zrn)• Allanite (Aln)• Xenotime

(Xen)

U-Th-REE Minerals

• Molybdenite• Apatite (Ap)• Titanite • Garnet• Rutile• Chalcopyrite• Pyrrhotite• Nb oxide• Graphite

Primary Minerals

Uraniferous pegmatites

Uraninite- and uranothorite-bearingAlso contain

zircon and minor allanite

Monazite is very rare

Typically intrude the western part of the fold nose

Th- and LREE Pegmatites

• Monazite-rich• Also contain

zircon, uranothorite-thorite, altered allanite, and xenotime as the main U-Th-REE hosts

• Rare Nb oxide

• Generally confined to eastern portions of the fold nose

Major element geochemistry

Legend

Alumina saturation index

Samples range from strongly peraluminous (off the chart) to slightly metaluminous

No distinct difference between U and Th mineralized pegmatites

S-type granitoids

Trace element geochemistry

Two major compositional subdivisions of pegmatites; correspond to mineralogical subdivisionsU- plus Th-richTh- and LREE-rich

Metamorphic Mineral Assemblages in host pelitic gneiss

Garnet Biotite Cordierite Sillimanite Spinel K-feldspar Quartz Plagioclase Rutile Myrmekite NO prograde

muscovite

Upper amphibolite to granulite facies metamorphism

Pegmatites – Partial melting at depth

vs. in-situ?

Migmatites associated with the granitic pegmatites

Leucosomes tend to be boudinaged, but also form small pegmatitic veins

Melt occasionally forms thin rims around minerals, and locally larger blobs

Biotite frequently shows degradation due to partial melting in thin section

Model for Fraser Lakes Zone B pegmatitesschematic mid-crustal cross-section

• Melts generated at depth (a)

• Transported upwards along the structural discontinuity/contact between Archean and Wollaston Group (b)

• Underwent igneous assimilation-fractional crystallization during transport and crystallization

• Melts were concentrated in antiformal fold noses (c)

• Peraluminous chemistry agrees with the pegmatites forming due to partial melting of pelitic gneisses

a

b

c

Similarities to Rössing/Rössing South

• Uranium mineralization is in late-tectonic to post-tectonic granitoids

• Structurally controlled mineralization -concentrated in antiformal fold noses

• Concentrated in areas of highest metamorphic grade

• Melt generated by partial melting of metasedimentary gneisses

Extract Resources, 2009

Modified from Ray, 1979

Conclusions

• Structurally controlled, granitic pegmatite-hosted U and Th mineralization (+/- REE mineralization)

• Hosted by Hudsonian granitic pegmatites intruding at/near the contact between Wollaston Group metasediments and Archean orthogneisses

• Regional metamorphism up to granulite facies

• Granitic pegmatites formed by partial melting of metasedimentary rocks in the middle to lower crust followed by transport and assimilation-fractional crystallization

• Similarities to Rössing and Rössing South granitoid-hosted U deposits

Acknowledgements JNR Resources Inc. – access to drill core,

outcrops, geological and geophysical datasets, logistical support, and financial assistance

NSERC - Discovery Grant (Ansdell)

University of Saskatchewan Graduate Scholarship (Austman)

Saskatchewan Research Council – geochemical analysis

Blaine Novakovski – thin section preparation

Kimberly Bradley – assistance with petrography

Steven Creighton and Tom Bonli – assistance with electron microprobe analyses