geocanada 2010 - austman et al - fraser lakes zone b
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PETROGRAPHY AND GEOCHEMISTRY OF GRANITIC PEGMATITE AND LEUCOGRANITE- HOSTED URANIUM &
THORIUM MINERALIZATION: FRASER LAKES ZONE B, NORTHERN SASKATCHEWAN, CANADA
AUSTMAN, Christine L.1, ANSDELL, Kevin M.1, and ANNESLEY, Irvine R.1,2
(1) Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2 (E-mail: [email protected]);
(2) JNR Resources Inc., Saskatoon, SK, Canada S7K 0G6
Abstract
References
Acknowledgements
Analytical Methods
Garnet, cordierite, sillimanite, pyroxenes, hornblende, and spinel in the surrounding metamorphic
rocks indicate that the regional metamorphism was of upper amphibolite to granulite facies
Abundant migmatites in drill core (Fig. 26) indicate that these metamorphic rocks underwent
partial melting, consistent with melt micro-textures seen in thin section (Fig. 27, 28)
Significant-sized granitoids of similar age and appropriate geochemistry in the area are not
common (i.e. typical of the middle crust)
Peraluminous to weakly metaluminous composition (Fig. 21, 23), plus the mineralogy of the granitic
pegmatites and leucogranites (quartz-feldspar-biotite-garnet) is consistent with an origin by partial melting of
metasedimentary rocks at variable depths within the middle to lower crust
Difference in U and Th contents of the granitic pegmatites could be due to a combination of factors, including
igneous assimilation and fractional crystallization (AFC) processes, interaction with magmatic fluids (B, F, Cl,
CO2, H2O), different melt-generating reactions, and variable source rock chemistry in the middle to lower crust
Fraser Lakes Zones A and B are located in JNR Resources Inc.’s
Way Lake Property, ~ 55 km from the Key Lake uranium mine in
the Athabasca Basin and ~25 km from the basin’s SE edge (Fig. 1)
Paleoproterozoic Wollaston Group metasedimentary rocks and
Archean orthogneisses underlie the study area, which was complexly
deformed, intruded, and metamorphosed during the Trans-Hudson
Orogen (~ 1.8 Ga)
Zone A is in a NE-plunging synformal fold nose and Zone B is in an
antiformal fold nose adjacent to a 65km long folded electromagnetic
(EM) conductor (Fig. 2, 4)
At Zone B, the uranium and thorium mineralization is located in a
~500 m x 1500 m area northwest of the Fraser Lakes (Fig. 2, 4)
Multiple generations of pegmatites including syn-tectonic
(subcordant to gneissosity, often radioactive) and post-tectonic
(discordant, non-mineralized) pegmatites (Austman et al. 2009)
E-W ductile-brittle and NNW– and NNE-trending brittle structures cross-cut Zone B (Annesley et al., 2009)
Preliminary U-Th-Pb chemical age dating of uraninite from one of the Fraser Lakes pegmatites yielded a
crystallization age of 1770 ±90 Ma, plus younger age clusters which can be correlated to U-mineralization
events in the Athabasca Basin (Annesley et al. 2010a)
Radioactive granitoids similar to the Fraser Lakes granitic pegmatites underlie several unconformity
uranium deposits of the eastern Athabasca Basin, including P-Patch, McArthur River Zone 2, Eagle
Point, Sue C, and Roughrider (Annesley et al., 2000a, 2000b, 2005, 2009, 2010b; Annesley and Madore,
1999; Madore et al., 2000; Portella and Annesley, 2000)
Radioactive granitoids are one of the potential sources of the uranium for the unconformity U deposits
Prior to erosion, the Athabasca sandstone/basement unconformity was ~ 200-250 m above the present
outcrop surface, indicating the potential for unconformity U mineralization in the area (Annesley et al., 2009)
Located just outside of the Athabasca Basin, the Fraser Lakes uranium- and thorium- bearing granitic pegmatites and
leucogranites are one example of igneous-hosted uranium and thorium occurrences in the Wollaston Domain of northern
Saskatchewan. The mineralized granitic pegmatites and leucogranites intrude the highly deformed contact zone between
Wollaston Group metasedimentary rocks and underlying Archean orthogneisses. Whole rock geochemical analyses of
drill core samples from Zone B indicates the presence of multiple groups of granitic pegmatites that underwent igneous
assimilation-fractional crystallization (AFC) processes. The granitic pegmatites generally fall within Černý and Ercit’s
(2005) Abyssal-U and Abyssal-LREE pegmatite subclasses, and include syn-tectonic and post-tectonic varieties. The
granitic pegmatites are generally S-type and A-type granitoids that formed by partial melting of the intrusive hosts. Al-
teration of these pegmatites may have led to the remobilization of uranium and the development of unconformity-type
uranium mineralization in the Fraser Lakes area.
The purpose of this M.Sc. study is to develop a metallogenetic model for the Fraser Lakes deposits,
and clarify their relationship with the rich uranium deposits in the Athabasca Basin.
Fig. 1 Location of JNR’s properties in northern
Saskatchewan, including the Way Lake Property
(modified from map on JNR Resources Inc.
website).
Fig. 2 Topographic map showing the
location of Fraser Lakes Zones A and B, the
folded EM conductor (red dots), drill hole
collars (black dots), swamps (light green),
and lakes and rivers (blue).
Fig. 4 Total field aeromagnetic image of the
Fraser Lakes area. The EM conductor
corresponds to an aeromagnetic low (blue
to green colors). The black dashed lines are
basement lineaments/structures. Note the
location of Fig. 6 and Fig. 11.
The authors acknowledge the financial support of JNR Resources Inc., NSERC (Discovery Grant to Ansdell) and the University of Saskatchewan (Graduate Scholarship to Austman). Thanks to Blaine Novakovski for preparing the thin sections, to Kimberly Bradley from JNR
Resources Inc. for her assistance with petrography, and the Saskatchewan Research Council for the geochemical results.
Drill core from the Fraser Lakes Zone B deposit was examined for this study, with samples taken from
several drill holes for petrographic study. After drilling, each hole was probed using a gamma-ray probe to
test for radioactivity. Whole rock geochemical analysis (by ICP-MS and ICP-OES) of selected samples from
WYL-09-50, WYL-09-49, WYL-09-46, and WYL-09-525 was completed by the Saskatchewan
Research Council Geoanalytical Laboratories in Saskatoon.
Introduction
The radioactive granitic pegmatites, leucogranites and migmatitic leucosomes intrude the highly deformed contact between Archean orthogneisses and the overlying Wollaston Group (Fig. 6, 11)
Zoning (Fig. 5) is common, due to igneous assimilation-fractional crystallization (AFC) processes
Variable primary mineralogy, including quartz, feldspar, biotite, ± garnet, ± magnetite,
± ilmenite, ± titanite, ± muscovite, ± apatite, ± fluorite, ± sulphides, ± zircon, ± U-Th-REE-bearing
accessory minerals which vary depending on the composition of the pegmatite (See below for the
different kinds of pegmatites; also Fig. 7-10, 12-15, 17-19)
Accessory mineral assemblage is dependent on host rocks (example: magnetite is found only in
pegmatites intruded into the Archean orthogneisses) and the melt composition (see below)
The pegmatites in the western part of the fold nose tend to be enriched in both U and Th (Fig. 6-
10) with low Th/U ratio, while the pegmatites in the eastern part of the fold nose tend to have high
Th/U ratios, and show Th– and LREE-enrichment (Fig. 11-15)
Fig. 5 Drill core from WYL-09-50 showing fractionation from quartz-rich to feldspar-rich
in the core of this radioactive granitic pegmatite(158.7 - 62.7m).
Secondary pyrite is commonly found in altered uranium and thorium minerals Hematite, chlorite, clay, fluorite, silica, and
calcite alteration associated with weak to locally strong brittle fracturing (Fig. 16-19) Alteration indicates that there was post-
crystallization hydrothermal fluid flow
through the rocks, which may have caused remobilization of the U, Th, and REEs Possibly related to Athabasca basinal brines
Mineralogy
Fig. 26 Drill core from
WYL-09-524 (~15.6-19.8
m) with boudinaged
crustal melt pods and
radioactive granitic
pegmatites.
Conclusions Structurally controlled, basement-hosted U, Th, and LREE mineralization in Hudsonian-aged leucogranites and granitic pegmatites intruded into the highly deformed contact
between Paleoproterozoic graphitic pelitic gneisses and Archean orthogneisses
The pegmatites on the northern limb of the fold nose are Th– and LREE-enriched, becoming U– and HREE-enriched on the western side of the fold nose
Granitic pegmatites are of Černý and Ercit’s (2005) Abyssal-U and Abyssal-LREE subclasses, and formed by partial melting of the Wollaston Group
Post-crystallization alteration and fluid flow through the rocks raises the possibility of remobilization of U, Th, and REE’s
Fraser Lakes U-deposits are similar to several basement-hosted U-deposits in the Athabasca Basin, and to the pegmatite-hosted U deposits in the Grenville Province
The potential exists for finding basement-hosted unconformity-type mineralization in the Fraser Lakes area
Future work to include: additional petrography, electron microprobe work, further whole-rock geochemical analysis, Pb-isotope studies, XRF analysis, and U-Pb chemical age
dating of the mineralization to aid in the development of a metallogenetic model and examination of the potential for future discoveries
Annesley, I.R. & Madore, C., 1999, Leucogranites and pegmatites of the sub-Athabasca basement, Saskatchewan: U protore? Mineral Deposits: Processes to Processing (Stanley, C.J. et al., eds.), Balkema 1: 297-300.
Annesley, I., Madore, C., Kusmirski, R., and Bonli, T., 2000, Uraninite-bearing granitic pegmatite, Moore Lakes, Saskatchewan: Petrology and U-Th-Pb chemical ages. In: Summary of Investigations 2000, Volume 2, Saskatchewan Geological Survey, Saskatchewan Energy and
Mines, Miscellaneous Report 2000-4.2. p. 201-211.
Annesley, I.R., Madore, C. and Portella, P., 2005, Geology and thermotectonic evolution of the western margin of the Trans-Hudson Orogen: evidence from the eastern sub-Athabasca basement, Saskatchewan, Canadian Journal of Earth Sciences, 42, 573-597.
Annesley, I., Cutford, C., Billard, D., Kusmirski, R., Wasyliuk, K., Bogdan, T., Sweet, K., and Ludwig, C., 2009, Fraser Lakes Zones A and B, Way Lake Project, Saskatchewan: Geological, geophysical, and geochemical characteristics of basement-hosted mineralization.
Proceedings of the 24th International Applied Geochemistry Symposium (IAGS), Fredericton, NB. Conference Abstract Volume 1. p. 409-414.
Annesley, I.R., Creighton, S., Mercadier, J., Bonli, T., and Austman, C.L., 2010a, Composition and U-Th-Pb chemical ages of uranium and thorium mineralization at Fraser Lakes, northern Saskatchewan, Canada. GeoCanada 2010, Calgary, Canada, May 2010.
Annesley, I.R., Wheatley, K., and Cuney, M., 2010b, The Role of S-Type Granite Emplacement and Structural Control in the Genesis of the Athabasca Uranium Deposits. GeoCanada 2010, Calgary, Canada, May 2010, Extended Abstract.
Austman, C.L., Ansdell, K.M., and Annesley, I.R., 2009, Granitic pegmatite- and leucogranite-hosted uranium mineralization adjacent to the Athabasca basin, Saskatchewan, Canada: A different target for uranium exploration. Geological Society of America Abstracts with Programs,
Vol. 41, No. 7, p. 83.
Boynton, W.V., 1984. Cosmochemistry of the rare earth elements: meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63–114.
Černý, P., and Ercit, T. 2005, The classification of granitic pegmatites revisited. The Canadian Mineralogist, 43, 2005-2026.
Frost, B.R., Arculus, R.J., Barnes, C.G., Collins, W.J., Ellis, D.J., Frost, C.D., 2001, A geochemical classification of granitic rocks. Journal of Petrology, 42, 2033–2048.
Kretz, R., 1983, Symbols for rock-forming minerals. American Mineralogist, 68, 277-279.
JNR Resources Inc., 2009, —Home Page—Oct. 10, 2009, Saskatoon, 10/10/2009, http://www.jnrresources.com.
Lentz, D., 1991, U-, Mo-, and REE-bearing pegmatites, skarns and veins of the Grenville Province, Ontario and Quebec. Can. Journal of Earth Sciences, 28, 1-12.
Madore, C., Annesley, I. and Wheatley, K., 2000, Petrogenesis, age, and uranium fertility of peraluminous leucogranites and pegmatites of the McClean Lake / Sue and Key Lake / P-Patch deposit areas, Saskatchewan. GeoCanada 2000, Calgary, Alta., May 2000, Extended Abstract
1041 (Conference CD).
Portella, P. and Annesley, I.R., 2000a, Paleoproterozoic tectonic evolution of the eastern sub-Athabasca basement, northern Saskatchewan: Integrated magnetic, gravity, and geological data. GeoCanada 2000, Calgary, Alta., May 2000, Extended Abstract 647 (Conference CD).
Portella, P. and Annesley, I.R., 2000b, Paleoproterozoic thermotectonic evolution of the eastern sub-Athabasca basement, northern Saskatchewan: Integrated geophysical and geological data. in Summary of Investigations 2000, Volume 2: Saskatchewan Geological Survey,
Saskatchewan Energy and Mines, Miscellaneous Report 2000-4.2, 191-200.
Shand, S., 1943, The Eruptive Rocks, 2nd ed., New York: John Wiley, 444 pp. Fig. 16 Granitic pegmatite (96.8 m) in WYL-09-
41 with hematite alteration, fracture controlled
pyrite and chlorite, and up to 2100 cps
radioactivity.
Fig. 19 Granitic pegmatite (WYL-
09-50-166.2) with hematite,
fluorite, calcite, and epidote
alteration in fractures.
Fig. 17 Moderate to strongly
altered granitic pegmatite (WYL-
09-50-215.8) containing zircon,
and possibly allanite.
Fig. 18 Granitic pegmatite from
WYL-09-50 (215.8 m) with calcite-
fluorite-quartz veining and altered
feldspar.
Alteration
Fig. 7 (PPL), 8
(XPL); Granitic
pegmatite from
WYL-09-50 (~191.6
m) with abundant
zoned zircon (Zrn),
apatite (Ap), and
monazite (Mnz) in a
cluster of biotite
(Bt). Abbreviations
after Kretz (1983).
Fig. 9 (PPL), 10
(RL); Disseminated
fine grained
uraninite (Urn) in a
pleochroic halo
around an altered
allanite (Aln) grain
in a granitic
pegmatite from
WYL-09-50 (~
232.9 m).
U– and Th-enriched pegmatites (western part of the fold nose) Th– and REE-rich granitic pegmatites (eastern part of fold nose)
Fig. 11 Cross-section from
the northern limb of the
fold nose at Zone B. Drill
holes include WYL-09-
43a, -43, -44, -45, and -46.
Note the increase in
radioactivity in the
granitic pegmatites with
local increases in pelitic
gneiss, granite gneiss, and
orthogneiss intervals. See
Fig. 4 for the location of
the cross-section.
Fig. 6. Cross-section from the
western limb of the fold nose at
Zone B. Drill holes include
WYL-09-41, -42, -49, and -50.
Note the increase in
radioactivity (blue line – gamma
probe results) in the granitic
pegmatites (red units) with local
increases in pelitic gneiss (green)
and Archean orthogneiss and
granitic gneiss (orange and
pink) intervals. See Fig. 4 for
the location of the cross-section.
Fig. 12, 13. Biotite-
rich section of a
granitic pegmatite
(WYL-09-46-36.1)
with hematized
monazite (Mnz),
uranothorite-thorite
(Thr) containing
pyrite (Py) inclusions,
and zoned zircon
(Zrn).
Fig. 14 (WYL-09-46-
83.0), 15 (WYL-09-46-
42.8) Pegmatites with
monazite (Mnz), zir-
con (Zrn), ilmenite
(Ilm), magnetite (Mgt),
and titanite (Ttn).
Monazite is being
altered to hematite
(Hem), chlorite (Chl),
and clay.
Fig. 20 Major element (TiO2, Al2O3, FeOt, MgO,
CaO, Na2O, K2O and P2O5; all in wt. %) and trace
element (Ba, Rb, Sr, Zr, Th/U, and Y; all in ppm)
Harker diagrams. Some of the elements (Al2O3, CaO,
Na2O, K2O, and Sr) show weak trends likely related
to igneous assimilation-fractional crystallization
processes. The Zr content is mainly controlled by zir-
con, while Y is mostly related to the presence of al-
lanite and/or garnet. These pegmatites have higher
SiO2 contents than the Th-rich pegmatites and have
variable chemistry suggesting that the data could be
from multiple groups of pegmatites. The spread in
the data also could be due to chemical zonation
within individual pegmatites.
Fig. 21 Classification
plots after Frost et al.
(2001) and Shand
(1943) showing that
these pegmatites are
more magnesian
relative to the Th-rich
pegmatites. The
pegmatites are also
weakly metaluminous
to peraluminous in
composition.
Fig. 22 Major and trace element Harker diagrams
for the Th– and LREE-rich pegmatites. These
pegmatites show strong P2O5 enrichment due to
monazite; Th enrichment due to monazite,
uranothorite-thorite, and allanite; Zr enrichment
due to zircon; and strong Y anomalies due to
allanite and/or garnet. These pegmatites have lower
SiO2 contents than the other pegmatites (with the
lowest SiO2 contents being in samples containing
abundant garnet). Weak trends in the K2O and Ba
data are suggestive of igneous assimilation-
fractional crystallization processes.
Fig. 27, 28 Garnetiferous
pelitic gneiss (WYL-09-44-
61.4) with melt micro-textures
at the contact between garnet
and biotite. Biotite is being
consumed in the melt-
generating reaction.
Fig. 3 Aerial photograph (looking to the
northeast) of the Fraser Lakes Zone B area,
showing the swamp corresponding to the
surface trace of the EM conductor.
U-Th-REE minerals: monazite, zircon, allanite, and members of the
uranothorite-thorite solid solution series
Mineralogy is indicative of Černý and Ercit’s (2005) Abyssal-LREE subclass
Fig. 25 Chondrite-normalized (Boynton 1984)
REE plot showing the differences in REE contents
of the different pegmatites. The Th– and LREE-
rich pegmatites show strong enrichment in the
LREEs and weak enrichment of most of the
HREEs relative to the other pegmatites. The
strong negative Eu-anomaly of the Th– and LREE
-rich pegmatites is likely due to plagioclase frac-
tionation. The LREE enrichment in the Th– and
LREE-rich pegmatites is indicative of Černý and
Ercit’s (2005) Abyssal-LREE subclass.
U-Th-REE minerals: zircon, allanite, and uraninite
Mineralogy is indicative of Černý and Ercit’s (2005) Abyssal-U subclass
Fig. 23
Classification plots
after Frost et al.
(2001) and Shand
(1943) showing that
the Th-rich
pegmatites are more
iron-rich than the
majority of the other
pegmatites and are
peraluminous in
composition.
All pegmatites (mineralized and unmineralized)
Fig. 24 U (ppm) vs. SiO2 (wt.
%) and Th (ppm) vs. SiO2
(wt. %) plots of mineralized
and unmineralized
pegmatites. The uranium and
thorium mineralization in the
Fraser Lakes pegmatites is
characteristic of Černý and
Ercit’s (2005) Abyssal-U class
of granitic pegmatites.
Origin of the granitic pegmatites and primary mineralization
Geochemistry
Highly Th– and LREE-enriched pegmatites (high Th/U) Other Pegmatites (U– and Th– enriched plus non-enriched samples)