McKechnie and Annesley GSA 2014 Presentation
-
Upload
christine-mckechnie -
Category
Documents
-
view
69 -
download
0
Transcript of McKechnie and Annesley GSA 2014 Presentation
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?