Fraser Lakes Zone B mineralized granitic pegmatites and their pelitic hosts: Mineral chemis-

try, stoichiometry, and geothermobarometry

AUSTMAN, Christine L.1

, ANNESLEY, Irvine R.1,2

, and ANSDELL, Kevin M.1

(1) Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK Canada S7N 5E2 (E-mail: christine.austman@usask.ca);

(2) JNR Resources Inc., Saskatoon, SK, Canada S7K 0G6

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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

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Annesley, I.R., Creighton, S., Mercadier, J., Bonli, T., and Austman, C.L., 2010, Composition and U-Th-Pb chemical ages of uranium and thorium mineralization at Fraser Lakes, northern Saskatchewan, Canada: GeoCanada 2010, Calgary, Canada, May 2010, Extended Abstract.

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Lentz, D. 1992, Petrogenesis of U-, Th-, Mo- and REE-bearing Pegmatites, Skarns, and Veins in the Central Metasedimentary Belt of the Grenville Province, Ontario and Quebec. Ph.D. thesis, University of Ottawa, Ottawa, Ontario.

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Acknowledgements The authors acknowledge the financial support of JNR Resources Inc., NSERC (Discovery Grant to Ansdell) and the University of Saskatchewan (Department Heads Research Grant to Ansdell and Graduate Scholarship to Austman). Thanks to Blaine Novakovski for preparing the thin sections, to Kimberly Bradley from JNR Re-

sources Inc. for her assistance with petrography, and Steven Creighton for his assistance with the electron microprobe work.

Purpose: to obtain estimates of the pressures and temperatures of regional metamorphism in the area

and determine the relationship of regional metamorphism to pegmatite intrusion.

Conclusions The host rocks to the Fraser Lakes granitic pegmatite-hosted U-Th-REE

mineralization underwent regional metamorphism up to lower granulite

facies, based on the presence of key indicator minerals including spinel,

sillimanite, and cordierite in the pelitic gneisses, and orthopyroxene in the

Archean orthogneisses

Relict kyanite in the pelitic gneisses indicates that the rock experienced

conditions that enabled it to form during the prograde part of its P-T path

The maximum pressure that the pelitic gneisses experienced during

metamorphism was up to ~7.6 kbar at temperatures of up to ~ 760 °C

Pegmatite crystallization began at 750 - 830 °C with the oxide minerals, and

feldspars and quartz crystallizing later at lower temperatures (>500 °C for

similar pegmatites in the Grenville Province (Lentz 1992))

Similarity of pegmatite intrusion temperatures to maximum metamorphic

temperatures agrees with other evidence (Austman et al. 2009, 2010 a, b)

that the mineralized granitic pegmatites formed by partial melting of pelitic

gneisses ± orthogneisses during regional metamorphism

Future work will include additional microprobe analyses of pegmatites

(biotite, magnetite-ilmenite, feldspar) and pelitic gneisses (cordierite, spinel,

garnet, biotite) in order to further constrain pressures and temperatures of

regional metamorphism and pegmatite crystallization

Location of the study area

Regional Geology

Area is underlain by Archean orthogneisses,

Wollaston Group metasedimentary rocks (pelitic

gneisses ± graphite, psammopelitic gneisses, and

calc-silicate gneisses), and Hudsonian intrusives

(Fig. 2) that were complexly deformed and

metamorphosed during the Trans-Hudson

Orogen ~1.8 Ga (Annesley et al. 2009)

Two mineralized zones, A and B (see Fig. 2), are

hosted by NE-plunging regional fold structures

adjacent to a 65 km long folded electromagnetic

(EM) conductor (Annesley et al. 2009)

At Zone B, the uranium and thorium

mineralization is located in a ~500 m x 1500 m

area NW of the Fraser Lakes in an antiformal

fold nose cross-cut by E-W ductile-brittle and

NNW- and NNE-trending brittle structures (Fig. 2,

3, Annesley et al. 2009)

Multiple generations of pegmatites including

syn-tectonic (subcordant to gneissosity, often

radioactive) and post-tectonic (discordant, non-

mineralized) pegmatites intrude the Archean-

Wollaston Group contact (Austman et al. 2009)

Pegmatites in the western part of the fold nose are

U and Th-enriched (Th/U ~1; up to 0.242% U3O8 with

0.254% ThO2 over 0.5 m in drill core), while those in

the eastern part are Th + LREE-enriched with vari-

able U (Th/U ~ 2-20; up to 0.109% ThO2 with 0.013%

U3O8 over 7.0 m in drill core) (JNR Resources Inc.,

2010)

The presence of migmatites, melt-textures in thin

section, geochemical trends between the pegmatites and

pelitic gneisses, and initial U-Th-Pb chemical age

dating of the pegmatites (1795 ± 15 Ma, Annesley et al.

(2010)) suggests that the pegmatites formed by partial

melting of pelitic gneisses ± Archean orthogneisses in

the middle to lower crust during the Trans-Hudson

Orogen (Austman et al. 2009, 2010 a, b)

Methodology Several samples of Fraser Lakes Zone B drill core and outcrop were prepared for petrological studies. Of these samples, one

granitic pegmatite (WYL-10-61-190.3) and two pelitic gneiss thin sections (WYL-09-49-36.1; WYL-09-37.5) were

selected for initial electron microprobe analysis and conventional geothermobarometry. All microprobe analyses were com-

pleted at Saskatchewan Research Council’s Geoanalytical Laboratories using a Cameca SX-100 electron microprobe. Pres-

sure and temperature constraints are given by a combination of mineral assemblage data and conventional

geothermobarometry.

Fraser Lakes Zones A and B are located in JNR Resource’s Way

Lake Property (Fig. 1 - modified map from JNR Resources Inc.,

2010) in northern Saskatchewan, ~ 25 km from the SE edge of the

Athabasca Basin and ~ 55 km from the Key Lake Uranium Mine

Fig. 3. Geological map of the Fraser Lakes Zone B area showing the presence of multiple

pegmatites (red and blue) at/near the contact between Archean orthogneisses (orange and

purple) and Wollaston Group metasedimentary rocks (green). Modified from Ko, 1971.

Abstract The Fraser Lakes Zone B U-Th-REE mineralized granitic

pegmatites intruded into the highly deformed contact

between the Wollaston Group pelitic gneisses and

Archean orthogneisses during the Trans-Hudson Orogen

~1.8 Ga. The host pelitic gneisses and orthogneisses were

metamorphosed to lower granulite facies, at a maximum

temperature of about 765 °C and maximum pressure of

around 7.5 kbar, while the granitic pegmatites were

intruded at a temperature of 700- 800°C. The pelitic

gneisses later underwent at least one period of retrograde

metamorphism at approximately 700-730 °C (5.6-7.1 kbar

pressure). The similarity of intrusion temperatures to

metamorphic temperatures agrees with other evidence

which suggests that intrusion of the pegmatites was

related to regional metamorphism and migmatization of

the Wollaston Group in the Fraser Lakes area.

Fig.1

Mineral Assemblages

Pelitic Gneisses Granitic Pegmatites

Fig. 2 Regional geological map of the Fraser Lakes area. Modified from Ray (1980).

Major minerals

(primary): quartz, biotite,

plagioclase, and k-feldspar

Other primary minerals:

± titanomagnetite

(pegmatites in Archean

orthogneisses only), ±

ilmenite, ± pyrite, ±

chalcopyrite, ± rutile, ±

titanite, ± fluorite, ±

molybdenite, ± apatite, ±

garnet, ± graphite,

pyrrhotite, ± pentlandite

Radioactive accessory

Minerals (primary): ±

uraninite, ± uranothorite,

± zircon, ± monazite, ±

allanite

Alteration assemblage: ±

chlorite, ± muscovite, ±

clay minerals, ± hematite,

± pyrite, ± fluorite, ±

galena

Metamorphic minerals:

Plagioclase, quartz, biotite,

± k-feldspar, ± garnet, ±

cordierite, ± sillimanite, ±

graphite, ± ilmenite, ±

spinel, ± kyanite, ± zircon,

± monazite, ± uraninite, ±

apatite, ± pyrite, ± other

sulphides and oxides

Alteration assemblage:

± chlorite, ± muscovite, ±

clay minerals, ± hematite, ±

pyrite, ± fluorite, ± rutile

Sample # Rock type Methods Maximum

Pressure

(core)

Retrograde

Pressures (rim)

Average max T°C

(core)

Retrograde T°

C (rim)

Max T°C of

intrusion

log10 f(O2)

WYL-09-49

-36.1

Sill-Gt-Bt-

bearing pelitic

gneiss

Ti-Bt(T1), Gt-Bt

(T2), GASP (P1,

P2)

~7.2 kbar

(@ 765 °C,

P1);

~5.6 kbar (@

702 °C, P1);

765 °C (T1) ~ 700 °C (T1) n/a n/a

WYL-09-50

-37.5

Sill-Crd-Spl-Gt-

Bt-Gr-bearing

pelitic gneiss

Ti-in-Bt(T1), Gt-

Bt(T2), GASP

(P1, P2)

~7.6 kbar

(@ 750 °C,

P1)

~6.8 -7.1 kbar

(@709 °C - 730

°C, P1)

750 °C (T1) 709 - 730 °C

(T1)

n/a n/a

WYL-09-61

-190.3

Magnetite-

bearing Granitic

Pegmatite

Mgt-Ilm (T2,

T3), f(O2) (1, 2)

n/a n/a n/a n/a ~ 831 °C (T2);

706 - 828°C

(T3)

~ -1.92 relative to

NNO (f1);

-17.99 to -15.18 (f2)

Models:

T1: Titanium-in-Bt

geothermometer of Henry

(2005); applied to non-graphitic

pelites by Eric (2009).

T3, f(O2)2:

Magnetite-ilmenite geothermometer

and f(O2) geobarometer of Anderson

and Lindsley (1985) calculated using

ILMAT (Lepage 2003). The large

range of temperatures is due to

differences in the calculation of the

molecular fraction of ulvospinel and

ilmenite.

P1: GASP1.EXE program available from Ganguly

(2010), which uses thermodynamic properties from

Berman (1988) and experimental data from Koziol and

Newton (1988) for the pure end-member GASP reaction.

Thermodynamic solution models are from Ganguly et al.

(1996) (garnet) and Elkins and Grove (1990)

(Plagioclase). Ti-in-biotite temperatures (T1) from

co-existing biotite were used in the calculations.

T2, f(O2)1:

Magnetite-Ilmenite geother-

mometer and f(O2) geobarometer

of Ghiorso and Evans (2008); f

(O2) is relative to NNO of O'Neill

and Pownceby (1993).

Observed Mineral Reactions in Pelitic Gneisses

Results of Initial Conventional Geothermobarometry

Fig. 12.

Boudinaged

felsic melt pods

with garnet

cores from

WYL-09-37

(~190.0 m).

Fig. 14. Pelitic gneiss (WYL-09-50-

37.5) with quartz, plagioclase,

biotite , spinel (Spl), sillimanite,

cordierite (Crd), graphite (Gr), and

monazite. This thin section also

contains garnet and k-feldspar.

Fig. 5. Outcrop (Trench 2) of a typical radioactive

granitic pegmatite showing coarse grain size and

plagioclase- (Plag), biotite- (Bt), and quartz- (Qtz)

rich mineralogy. Abbreviations after Kretz (1983).

Fig. 4. Radioactive granitic pegmatite from drill hole

WYL-09-44. Pegmatite shows zoning, from a

plagioclase-rich core through k-feldspar and quartz-

rich zones.

Fig. 11. Drill core from WYL-10-61 showing

compositional variation in the pelitic gneiss

(including graphite-cordierite-sillimanite-rich

layers) and local boudinaged felsic melt pods.

Fig. 13. Pelitic gneiss (WYL-09-49-36.1) con-

taining biotite, garnet, sillimanite, monazite,

quartz, and feldspar.

Fig. 15. Pelitic Gneiss (WYL-10-61-78.1)

with altered cordierite, biotite, sillimanite,

feldspar, and quartz

Fig. 16. Pelitic gneiss (WYL-09-49-)

contains sillimanite after kyanite,

with some remnant sillimanite left.

Fig. 17. Qualitative P-T

diagram for the pelitic

gneisses from the Fraser

Lakes Zone B area

showing an approximate

clockwise metamorphic P

-T path based on mineral

assemblages and

reactions observed in

thin section. Numbers

on the diagram

correspond to the

numbered reactions in

the adjacent table of

observed reactions.

(Diagram is modified

from Fig. 5 of Annesley

et al. 2005).

Fig. 7.

Biotite (Bt),

zircon

(Zrn), and

uraninite-

rich

granitic

pegmatite

(sample

Trench 2-2).

Fig. 6. Granitic

pegmatite

(WYL-10-61-

190.3) with

magnetite

(Mgt), ilmenite

(Ilm), and

uraninite (Urn)

intrusive into

Archean

orthogneisses.

Fig. 10. Arial view (looking

NE) of the Fraser Lakes

Zone B area.

Fig. 8. Quartz- rich,

radioactive granitic

pegmatite (outcrop sample).

Note the radiation cracks

surrounding the altered

uraninite (Urn) grain.

Fig. 9 Biotite- and monazite-

rich pegmatite (WYL-09-46

-83.0) with thorite (Thr) and

zircon (Zrn).

1. Bt + Sil +Qtz = Grt +Kfs + melt

2. Bt + Sil +Qtz = Grt +Crd +Kfs + melt

3. Bt + Sil + Qtz = Pl + Grt + Kfs + melt

4. Bt + Sil + Qtz + Pl = Grt + Crd ± Kfs +melt

5. Bt + Sil + Qtz + Pl = Crd ± Kfs + melt

6. Grt + Sil = Crd + Qtz ± Kfs

7. Grt + Sil = Crd + Spl + Ilm

Selected Mineral Chemistry and Stoichiometry

Note: Mineral stoichiometry for all minerals was calculated using Andy Tindle’s mineral re-calculation Excel

spreadsheets (Tindle, 2010)

Sample: WYL-09-50-37.5 WYL-09-49-36.1

Mineral (Core/

Rim;

interpretation)

Bt

(matrix,

peak)

Bt

(matrix,

peak)

Bt (C adj.

Gt;

retrogr.)

Gt (C;

peak?)

Gt (R;

retro.)

Pl (C;

matrix)

Pl (R;

matrix)

Bt (in

gt;

peak?)

Bt (adj.

Gt;

retro.)

Gt (C;

peak)

Gt

(C2, ?)

Gt (R;

retro.)

PL (C;

matrix,

peak?)

Pl (R;

matrix,

retro)

SiO2 35.57 35.93 36.15 SiO2 37.89 38.27 SiO2 64.04 64.04 SiO2 36.32 35.80 SiO2 37.63 37.88 37.60 SiO2 60.78 61.47

TiO2 4.77 4.77 3.96 TiO2 0.03 0.01 TiO2 0.00 0.00 TiO2 5.23 3.49 TiO2 0.03 0.00 0.03 TiO2 0.00 0.00

Al2O3 17.74 17.68 17.62 Al2O3 21.49 21.87 Al2O3 22.52 22.48 Al2O3 17.62 18.42 Al2O3 21.57 21.52 21.49 Al2O3 24.07 24.09

FeO 17.09 17.73 17.13 FeO 34.09 32.91 FeO 0.06 0.05 FeO 16.41 18.23 FeO 34.06 34.04 34.02 FeO 0.04 0.02

MnO 0.03 0.03 0.00 MnO 0.57 0.51 MnO 0.00 0.00 MnO 0.02 0.04 MnO 1.43 1.40 1.52 MnO 0.00 0.00

MgO 9.64 9.83 10.71 MgO 4.02 4.84 MgO 0.00 0.00 MgO 10.70 9.29 MgO 3.30 3.39 2.87 MgO 0.00 0.00

CaO 0.00 0.00 0.00 CaO 0.57 0.63 CaO 3.31 3.33 CaO 0.00 0.00 CaO 1.05 1.04 0.93 CaO 5.58 5.51

Na2O 0.14 0.12 0.14 Na2O 0.02 0.02 BaO 0.00 0.00 Na2O 0.32 0.17 Na2O 0.01 0.01 0.02 BaO 0.01 0.00

K2O 10.02 10.21 10.04 ZnO 0.02 0.01 Na2O 11.13 11.08 K2O 9.93 9.98 ZnO 0.01 0.01 0.01 Na2O 9.66 9.63

BaO 0.00 0.03 0.08 Cr2O3 0.02 0.02 K2O 0.26 0.22 BaO 0.05 0.07 Cr2O3 0.03 0.02 0.03 K2O 0.26 0.18

Cs2O 0.00 0.00 0.00 Y2O3 0.03 0.04 TOTAL 101.33 101.20 Cs2O 0.01 0.00 Y2O3 0.00 0.01 0.10 TOTAL 100.41 100.90

F 0.60 0.46 0.59 V2O3 0.00 0.02 F 1.24 1.16 V2O3 0.03 0.01 0.00

Cl 0.02 0.02 0.02 TOTAL 98.76 99.14 Si 11.23 11.24 Cl 0.08 0.07 TOTAL 99.14 99.34 98.61 Si 10.83 10.87

Li2O* 0.66 0.76 0.82 Al 4.66 4.65 Li2O* 0.87 0.72 Al 5.05 5.02

H2O* 3.69 3.81 3.75 Si 3.02 3.02 Ti 0.00 0.00 H2O* 3.47 3.43 Si 3.01 3.02 3.02 Ti 0.00 0.00

O=F,Cl 0.26 0.20 0.25 Al iv 0.00 0.00 Fe 0.01 0.01 O=F,Cl 0.54 0.50 Al iv 0.00 0.00 0.00 Fe 0.01 0.00

Total 99.71 101.20 100.76 Al vi 2.03 2.05 Mn 0.00 0.00 Total 101.73 100.37 Al vi 2.04 2.03 2.05 Mn 0.00 0.00

Ti 0.00 0.00 Mg 0.00 0.00 Ti 0.00 0.00 0.00 Mg 0.00 0.00

Si 5.35 5.34 5.38 Cr 0.00 0.00 Ca 0.62 0.63 Si 5.33 5.37 Cr 0.00 0.00 0.00 Ca 1.07 1.05

Al iv 2.65 2.66 2.62 Fe2+ 2.35 2.26 Na 3.78 3.77 Al iv 2.67 2.63 Fe2+ 2.35 2.34 2.38 Na 3.33 3.30

Al vi 0.50 0.44 0.46 Mn 0.04 0.03 K 0.06 0.05 Al vi 0.39 0.63 Mn 0.10 0.09 0.10 K 0.06 0.04

Ti 0.54 0.53 0.44 Mg 0.48 0.57 Ba 0.00 0.00 Ti 0.58 0.39 Mg 0.39 0.40 0.34 Ba 0.00 0.00

Fe 2.15 2.20 2.13 Ni 0.00 0.00 Total 20.36 20.34 Fe 2.02 2.29 Ni 0.00 0.00 0.00 Total 20.34 20.29

Mn 0.00 0.00 0.00 Zn 0.00 0.00 Mn 0.00 0.01 Zn 0.00 0.00 0.00

Mg 2.16 2.18 2.38 Ca 0.05 0.05 An (%) 13.93 14.10 Mg 2.34 2.08 Ca 0.09 0.09 0.08 An (%) 23.89 23.82

Li* 0.40 0.45 0.49 Total 7.98 7.98 Ab (%) 84.75 84.77 Li* 0.51 0.44 Total 7.99 7.98 7.98 Ab (%) 74.77 75.28

Ca 0.00 0.00 0.00 Or (%) 1.33 1.12 Ca 0.00 0.00 Or (%) 1.34 0.90

Na 0.04 0.04 0.04 Almandine 80.62 77.49 Na 0.09 0.05 Almandine 80.24 79.97 81.90

K 1.92 1.94 1.90 Andradite 0.00 0.00 K 1.86 1.91 Andradite 0.00 0.00 0.00

Sr 0.00 0.00 0.00 Grossular 1.60 1.77 Sr 0.00 0.00 Grossular 2.97 2.97 2.64

Ba 0.00 0.00 0.00 Pyrope 16.39 19.52 Ba 0.00 0.00 Pyrope 13.41 13.77 11.81

Cs 0.00 0.00 0.00 Spessartine 1.32 1.16 Cs 0.00 0.00 Spessartine 3.29 3.23 3.55

OH* 3.71 3.78 3.72 Uvarovite 0.07 0.05 OH* 3.40 3.43 Uvarovite 0.10 0.07 0.10

F 0.29 0.22 0.28 F 0.58 0.55

Cl 0.00 0.00 0.00 Cl 0.02 0.02

TOTAL 19.72 19.79 19.86 TOTAL 19.80 19.80

Sample: WYL-10-61-190.3

Mineral: Titanomagnetite

Ilmenite (inclusion,

primary)

SiO2 0.09 0.00

TiO2 17.84 49.79

Al2O3 9.12 0.01

Fe2O3 0.00 0.00

Cr2O3 0.01 0.00

FeO 70.56 44.81

MnO 2.08 4.04

MgO 0.08 0.00

CaO 0.00 0.00

ZnO 1.43 0.00

V2O3 0.05 0.06

NiO 0.04 0.00

TOTAL 101.30 98.71

Si 0.03 0.00

Al 3.27 0.00

Ti 4.08 1.94

Fe 17.95 1.94

Mn 0.54 0.18

Mg 0.04 0.00

Zn 0.32 0.00

Ca 0.00 0.00

Cr 0.00 0.00

Ni 0.01 0.00

V 0.01 0.00

Total 26.24 4.06

Pelitic gneisses

Pegmatites

Fig. 18

Fig. 19

Fig. 20

Fig. 21

Fig. 18-21. Backscatter electron images of the pelitic gneiss samples showing the location of

selected microprobe analysis points shown in the adjacent table. Fig. 18+ 19 are from

sample WYL-09-50-37.5 while Figs. 20 + 21 are from sample WYL-09-49-36.1.