Fraser Lakes Zone BPegmatite-hosted U-Th-REE

mineralization: Geology, geochemistry, and origin

Christine AustmanDepartment of Geological Sciences

University of Saskatchewan

Purpose Regional Geological Setting Fraser Lakes Geology Granitic Pegmatite Mineralogy and

Geochemistry Alteration and Remobilization of U-Th-Pb-

REE Metamorphism and Migmatization Metallogenetic Model Similarities to other uranium deposits Conclusions

Outline

Unresolved issues still exist in understanding uranium source, fluid flow, and the nature of the reductant involved in the formation of unconformity- related (U/C) uranium deposits

Determine if uraniferous pegmatites are related to high-grade unconformity-related uranium deposits of the Athabasca Basin (U protore?)

Determine whether granitic pegmatites represent a distinct target for uranium exploration in Saskatchewan

Purpose of this study

The Wollaston Domain consists of: Archean

orthogneisses (predominantly granitic gneisses)

Paleoproterozoic Wollaston Group metasedimentary rocks

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

Regional Geology

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

Johnson River

Granite

Inlie

r

Wolla

ston G

roup

met

ased

imen

tary

gnei

sses

Wollaston G

roup

metasedimentary gneisses

Fraser Lakes Zone B

Fraser Lakes Zone A

Needle Falls S

hear Zone

Wathaman

Batholith

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-08-526WYL-08-524WYL-08-526

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

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

Mineralogy

U-Th-REE Minerals• Zircon (Zrn)• Uraninite (Urn)• Uranothorite –

Thorite (Uth)

• Monazite (Mz)• 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?

Highly Variable!

Uraninite- and uranothorite-bearing◦ Also contain

zircon and minor allanite

◦ Monazite is very rare

Intrude the western part of the fold nose

Uraniferous pegmatites

Monazite-rich◦ Also contain

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

Generally confined to eastern portions of the fold nose

Th- and LREE Pegmatites

Alteration of granitic pegmatites

Retrograde Alteration• Chlorite (Chl)• Epidote (Ep)• Sericite (Ser) • Hematite (Hem)• Quartz (Qtz)

Hydrothermal Alteration• Fluorite (Fl)• Chlorite (Chl)• Hematite (Hem)• Clay minerals• Sausserite• Carbonate (Cal)• Quartz (Qtz)

Uranothorite - evidence for reaction (i.e. elevated Zr) where in contact with zircon

Radiogenically produced galena inclusions in U-Th-REE minerals

Uraninite and secondary uranium minerals in fractures with galena and pyrite

Monazite -commonly altered to hematite, chlorite, and clay

Alteration of U-Th-REE phases and remobilization of U, Th, Pb

Major element geochemistryLegend

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

S-type granitoids

Alumina saturation index

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

rich

Trace element geochemistry

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

muscovite

Metamorphic Mineral Assemblages – Pelitic gneisses

Upper amphibolite to lower granulite

facies metamorphism

◦ Clinopyroxene◦ Orthopyroxene◦ Hornblende◦ Biotite◦ Spinel◦ Myrmekite◦ Quartz◦ K-feldspar◦ Plagioclase

Metamorphic Mineral Assemblages –Orthogneisses

Upper amphibolite to lower granulite

facies metamorphism

Pegmatites – Partial melting at depth vs. in situ melting ?

Migmatites associated with the granitic pegmatites

Leucosomes tend to be boudinaged

Locally small pegmatitic veins of leucosome

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

Common near the contact between biotite and garnet and within K-feldspar

Biotite frequently shows degradation due to partial melting

Migmatites and melt-textures in the pelitic gneisses

Metallogenetic model – Fraser Lakes Zone B

a

b

c

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 to weakly metaluminous chemistry agrees with the pegmatites forming due to partial melting of pelitic gneisses

Uranium mineralization within late-tectonic to post-tectonic granitoids

Structurally controlled: mineralization concentrated in antiformal fold noses

Concentrated in areas of highest metamorphic grade

Formed from melts generated by partial melting of metasedimentary gneisses

Similarities to Rössing/Rössing South, among other deposits

Extract Resources, 2009

Modified from Ray, 1979

Radioactive leucogranites and granitic pegmatites are in the basement rocks under several Athabasca Basin unconformity uranium deposits (ex: McArthur River Zone 2, Millenium, Roughrider, and others)

Chlorite, clay, and hematite alteration with drusy quartz found in Zone B drill core is similar in appearance to that of basement-hosted unconformity uranium deposits

Similarities to Athabasca unconformity-type uranium deposits

Modified from Ray, 1979

Jefferson et al. 2007

~150 – 200 m

Structurally controlled, basement-hosted U and Th mineralization (+/- REE mineralization)

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

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

Regional metamorphism up to lower granulite facies

Granitic pegmatites experienced post-crystallization alteration and remobilization of U and Th

Similarities to Rossing and U/C-related basement-hosted uranium deposits

Conclusions

Acknowledgements Thesis co-supervisors: Dr. Irvine

Annesley and Dr. Kevin Ansdell

JNR Resources Inc. – access to drill core, geological and geophysical datasets, logistical support, and financial assistance

NSERC - Discovery Grant (Kevin)

University of Saskatchewan Graduate Scholarship

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