U-TH-REE-MINERALIZED GRANITIC PEGMATITES FROM FRASER LAKES ZONE

B: FERTILE CRUSTAL MELTS AND POTENTIAL U PROTORE?

McKechnie, Christine L. and

Annesley, Irvine R.

GSA 2014

Vancouver, B.C., Canada

Outline

• Geological Setting of Fraser Lakes Zone B

• Pegmatite geology

• Model for the Fraser Lakes Zone B U-Th-REE deposit

• Structural and Geochemical Controls

• Comparison w/ other pegmatite-hosted U deposits

• U protore?

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 boxMcKechnie et al. 2012 a, b, 2013

Fraser Lakes Geology• NE-SW regional fabric

• Two granite-/pegmatite-hosted U-Th-REE showings, Zones A and B, in the vicinity of Fraser Lakes

• 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

Fraser Lakes Geology

Modified from Ko, 1971

Granitic pegmatites and leucogranites• Granitic pegmatites and leucogranites

w/ variable amounts of quartz,

feldspars, biotite, and other minerals

• Inequigranular grain size distribution;

overall very coarse grained

(pegmatitic)

• Graphic intergrowths are common

• Variable width (cm to dm scale)

• Complexly zoned (igneous AFC

processes), zoning is variable between

pegmatites

• Multiple generations of pegmatites,

syn-tectonic (subcordant to

gneissosity, often radioactive) and

post-tectonic (discordant, non-

mineralized)

• Sharp contacts w/ host rocks

Mineralogy

U-Th-REE Minerals

• Uraninite (Urn)

• Thorite +/- U (Th)

• Monazite (Mz)

• Zircon (Zrn)

• 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!

* Magmatic and/or peritectic minerals

Group A vs. Group B Intrusives

Group 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

• U-Th-Pb chemical ages (uraninite) of 1.85-1.80 Ga

• Tend to be more Si-enriched (McKechnie et al. 2013)

• Abyssal-U (AB–U) subclass

Group B Intrusives

• Monazite-rich; i.e. Th + LREE-rich, w/ zircon (inherited cores), thorite, xenotime, allanite

• More “restite” minerals like Grt, Crd, Bt, etc.

• Monazite forms large clusters with biotite, is often partially resorbed

• Intruded the central part of the fold nose

• U-Th-Pb chemical ages (monazite) of 2.1 to 2.2 Ga, but field relationships suggest a similar age to the Group A intrusives

• LREE (AB–LREE) subclass

Granitic Pegmatites / Leucogranites –Possible relationship to partial melts

Migmatites in close association (i.e.

hosting) the radioactive intrusives

Leucosomes tend to be boudinaged, but

also form small pegmatitic veins

Crystallized melt? in thin section

Biotite frequently shows degradation due to

partial melting

► No nearby granite of similar age, yet field

relationships suggest that the migmatites are

possibly similar in age to the pegmatites.

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

(750 to 780°C, 6 to 8

kbar) @ ~1.8 Ga

Model for Fraser Lakes Zone B

McKechnie et al. 2012 b

• Later retrograde

metamorphism,

and associated

alteration due to

fluids moving

through the rocks

• (1) Melting of

source rocks at

depth 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

Structural controls

• Two main structural controls at Fraser Lakes Zone B:• (1) Archean-

Wolllaston Group contact• Sheared contact

• Rheological contrasts

• (2) Antiformal fold nose

Mercadier

et al. 2013

McKechnie

et al. 2012

b

Host rock Controls• Pegmatites intruding the

Archean gneisses contain magnetite and more K-spar

• No magnetite (only ilmenite), higher MgO/TiO2 ratios in pegmatites intruding the Wollaston Group metasediments

• More U concentrated at margins of pegmatites that are in contact with reduced lithologies (i.e. graphitic pelitic gneisses)• Similar to the redox control

proposed for the OrrefjellPegmatite-hosted Uranium Project in northern Norway

(Mikkel Vognsen, 2010 PDAC)

Geochemical/Mineralogical ControlsGroup A vs. Group B

• Differences in source rocks and degree of melting?• Group A - little to no monazite, uraninite-bearing (U-rich

source needed, U would have been concentrated in earlier melts)

• Group B - contain inherited monazite (most likely from the melt source based on size and age), no uraninite (so U-depleted source?), more “restite” minerals (i.e. melt generated from a more residual source)

• 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 U deposits

• Primary magmatic U mineralization with variable secondary overprint• Derived from partial melting of metasedimentary gneisses at depth during peak

thermal metamorphism; no relationship to any large granitic intrusions • Granitic to pegmatitic textures and “granitic” (sensu lato) compositions• Differences in composition and U concentration are likely due to different

sources, amount of transport and assimilation-fractional crystallization, and host rock composition

• Melts concentrated preferentially in antiformal fold noses and along shear zones as sheeted bodies, like at the Rössing and Husab (formerly Rössing South) deposits in Namibia

• Other similar pegmatites/granites are found in the Svecofennian Orogen of Norway and the Grenville Province; several other occurrences have also been found in Saskatchewan

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

U protore?• Been proposed (Annesley et al. 2000,

Mercadier et al. 2013, and others) that radioactive pegmatites may be a major source of U for unconformity-type U deposits in the nearby Athabasca Basin

• Chlorite, clay (including illite), and hematite alteration found in drill core

• Erosion at FLZB was 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

• No basement-hosted, unconformity-related mineralization has yet to be intersected during drilling activities in the area (but it may exist)

McKechnie

et al. 2013

Conclusions• Basement-hosted, magmatic U and Th mineralization (+/- REE

mineralization)

• Abyssal-class pegmatites (using Černý & Ercit 2005 classification scheme)

• 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, Orrefjell Uranium Project in Norway, and others

• Magmatic U mineralization may represent a new type of economic uranium deposit in northern Saskatchewan or protore for unconformity-releated U deposits

Questions?