ANATEXIS AND URANIUM PROTORE IN THE WOLLASTON DOMAIN, SASKATCHEWAN

McKechnie, Christine L. 1

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

1 Department of Geological Sciences, University of Saskatchewan2 JNR Resources Inc., Saskatoon, SK

Goldschmidt 2012June 2012

Outline• Geological Setting• Pegmatite Mineralogy and Geochemistry• Model for Granitic Pegmatite/Leucogranite

Generation• U protore?

The aim of this project was to determine whether these [granitic pegmatites and leucogranites]

represent a distinct target for uranium exploration in Saskatchewan and/or if the mineralization is somehow related to unconformity-type uranium

deposits

Regional Geology• Hearne Province • Deformed and metamorphosed

during the Paleoproterozoic (ca. 1.9-1.8 Ga) Trans-Hudson Orogeny (THO)

• ~ 25 km SE of the Mesoproterozoic Athabasca Basin

• 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

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

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Fraser Lakes Zone B

Fraser Lakes Zone ANeedle Falls

Shear Z

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Peter Lake D

omain

Fraser Lakes Geology

After Ko, 1971

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

Mineralogy Highly Variable!

* Magmatic and/or peritectic minerals

Group A and Group B Granitic Pegmatites/Leucogranites

Group A Intrusives• Contain abundant uraninite,

thorite, and zircon and minor allanite

• Intrude the western part of the antiformal fold nose

Group B Intrusives• Monazite-rich w/ zircon, thorite,

allanite, and xenotime• Intrude the central part of the

antiformal fold nose

Major element geochemistry

• Group A intrusives tend to be more Si-rich than Group B intrusives, with significant overlap

• Group B intrusives overlap with pelitic gneisses

• Controlled by sample mineralogy• (i.e. high SiO2 = quartz-rich;

low SiO2= higher mafics/oxide content)

• Controlled by host rock composition• Archean granitic

orthogneisses vs. Wollaston Group metasedimentary gneisses

Trace element geochemistry• The two groups

also have dissimilar trace element geochemistry related to their accessory mineral contents

• U- plus Th-rich (Group A)

• Th- and LREE-rich (Group B)

REE Geochemistry

Group A Intrusives• Generally flat or slightly

HREE-enriched patterns, low total REE content, variable Eu anomalies

Group B Intrusives• Generally show LREE

enriched patterns, higher total REE content, and strong –Eu anomalies

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

during THO

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 granitic pegmatites/leucogranite-hosted mineralizationschematic mid-crustal cross-sections

• Primary magmatic U (+/-Th, REE) mineralization within late-tectonic granitic pegmatites and leucogranites; 1850-1780 Ma (related to THO)

• Partial melting of metasedimentary gneisses (i.e. Wollaston Group equivalent) at depth during peak thermal metamorphism (THO)

• Melt migrated upwards along the structural discontinuity/contact between Archean and Wollaston Group, undergoing igneous assimilation-fractional crystallization (cross-section A)

• Melts concentrated preferentially in antiformal fold noses • Similarities to Rössing and Husab (formerly Rössing South) uranium deposits in

Namibia (cross-section B)

B) Extract Resources, 2009A) Modified from Ray, 1979

Granitoid-hosted U mineralization

after Cuney, 2005After Parslow and Thomas, 1982

Alteration of granitic pegmatites/leucogranites and remobilization of U, Th, PbPost-Crystallization Alteration (during cooling)•Chlorite (Chl)•Epidote (Ep)•Sericite (Ser) •Quartz (Qtz)

Hydrothermal Alteration•Fluorite (Fl)•Chlorite (Chl)•Hematite (Hem)•Clay minerals•Sausserite•Carbonate (Cal)•Quartz (Qtz)•With secondary hydrothermal U-Th-REE minerals

U protore?• Chlorite, clay (including illite), and

hematite alteration found drill core; similar to that of basement-hosted unconformity-type U deposits

• Erosion 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

• Drilling has yet to intersect a basement-hosted unconformity-type U deposit in the area – does not mean it does not exist

Modified from Ray, 1979

Sea

Basin 1.75-1.5 Ga

Basement 2.8 - 1.7 Gaore

1.4–1.1 Ga UO2

+/- REEs, Au, Cu, Co, Ni, As…

cf. presentations of Michel Cathelineau, 2011 and Mercadier et al., 2012

Unconformity

Evaporated Sea Water: high salinity fluids [Cl] Richard et al. (2011) Geochim. Comsochim. Acta 75, 2792-2810Mercadier et al., (2012) Geology

Mercadier et al. (2010)Lithos 115, 121-136

UO2 > 20%UO2 > 20%

× 105 !!![U]crust ~ 1.7 ppm

[U]ore ~ 20%

Up to 200.000 t U at approx. 20%: Giant uranium deposits of

high grade

Abundant U source (e.g. monazites, uraninites)

Hecht & Cuney (2000) Mineral. Deposita 35, 791-795

Unconformity-type uranium deposits: possible model

Salinity [U]: 10-6 to 10-2 mol/L, pH:3-4.5Richard et al. (2012), Nature Geoscience

percolation into the basement with leaching

Fraser Lakes Zone B

Conclusions• Structurally controlled, 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

• Similarities to Rössing and Husab (Rössing South) granitoid-hosted U deposits

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

• The magmatic U mineralization is potential protore for basement-hosted unconformity-type uranium deposits in the Fraser Lakes area (yet-to-be discovered) and elsewhere in the Wollaston Domain and Athabasca Basin

• Magmatic U mineralization may represent a new type of economic uranium deposit in northern Saskatchewan

Questions?