McKechnie et al. 2012 Goldschmidt talk
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Transcript of McKechnie et al. 2012 Goldschmidt talk
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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
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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
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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
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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 L
akes Gra
nite In
lier
Johnson River
Granite
Inlie
r
Wol
last
on G
roup
met
ased
imen
tary
gnei
sses
Wolla
ston G
roup
meta
sedimenta
ry g
neisses
Fraser Lakes Zone B
Fraser Lakes Zone ANeedle Falls
Shear Z
one
Peter Lake D
omain
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Fraser Lakes Geology
After Ko, 1971
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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
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Mineralogy Highly Variable!
* Magmatic and/or peritectic minerals
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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
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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
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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)
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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
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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
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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
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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
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Granitoid-hosted U mineralization
after Cuney, 2005After Parslow and Thomas, 1982
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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
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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
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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
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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
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Questions?