new u-pb zircon age dates from the tazin lake map area ... · quartz diorite fields on a...

Saskatchewan Geological Survey 1 Summary of Investigations 2007, Volume 2

New U-Pb Zircon Age Dates from the Tazin Lake Map Area (NTS 74N)

K.E. Ashton, C.D. Card, W. Davis 1, and L.M. Heaman 2

Ashton, K.E., Card, C.D., Davis, W., and Heaman, L.M. (2007): New U-Pb zircon age dates from the Tazin Lake map area (NTS 74N); in Summary of Investigations 2007, Volume 2, Saskatchewan Geological Survey, Saskatchewan Ministry of Energy and Resources, Misc. Rep. 2007-4.2, CD-ROM, Paper A-11, 8p.

Abstract Six new U-Pb zircon age dates are reported as a partial follow-up to recent mapping at selected localities in the Tazin Lake map area (NTS 74N) between 2003 and 2005. They confirm the presence of widespread ca. 2.6 Ga granitoid plutonism in the Nolan Domain and its southern extension into the northern Zemlak Domain. Circa 2.7 Ga granitoid plutonism is also recognized in the southwestern Zemlak Domain. Widespread magnetic orthogneisses in the western and northeastern Zemlak Domain are derived from ca. 2.3 Ga granitoids and were metamorphosed at 1.93 to 1.92 Ga. A non-magnetic intermediate pluton in the western Beaverlodge Domain was also emplaced at about 2.3 Ga.

Keywords: Tazin Lake, 74N, Zemlak Domain, Nolan Domain, Beaverlodge Domain, geochronology, Arrowsmith Orogen, Taltson Orogen.

1. Introduction This paper provides preliminary geochronological results for six recently analyzed samples from the Tazin Lake area (NTS 74N; Figure 1). Two of these samples were analyzed at the University of Alberta under the supervision of Larry Heaman; one utilizing conventional ID-TIMS techniques; the other laser ablation. Four other samples were analyzed using the Sensitive High Resolution Ion Microprobe (SHRIMP) at the Geological Survey of Canada in Ottawa under the supervision of Bill Davis.

2. Sample Description and Results from ID-TIMS and Laser Ablation Studies

a) Augen Granite, Western Zemlak Domain (Sample 4705-0607) A salmon pink augen granite from eastern White Bay in Harper Lake (Figure 2; UTM 573894 m E, 6613105 m N3) was sampled to determine a crystallization age from a zone of non-magnetic, variably mylonitized granitoids situated immediately east of known rocks of the Taltson Magmatic Suite (Ashton et al., 2005). The sample contained augen up to 2 cm in size (Figure 33 of Ashton et al., 2005), along with 9% chlorite after biotite, 2% late muscovite-sericite, 2% epidote, and minor titanite, allanite(?), apatite, and pyrite.

The augen granite was analyzed in thin section using the laser ablation technique at the University of Alberta. Of the resulting, moderately discordant analyses, a large number gave a weighted average 207Pb/206Pb age of 2707.0 ±3.3 Ma, which is interpreted as a minimum age and the best estimate for the crystallization age of the rock.

b) Milliken Lake Pluton, Southwestern Beaverlodge Domain (Sample 4703-0006) Although gabbros and ultramafic rocks are relatively common in the western Uranium City area, the Milliken Lake pluton is one of the few intermediate plutons encountered (Figure 3; UTM 626261 m E, 6593391 m N). It intrudes upper amphibolite facies, probable 3.0 Ga basement migmatites, although is not partially melted itself (Ashton et al., 2000). Representative samples plot at about the intersection of the granodiorite, tonalite, quartz monzodiorite, quartz diorite fields on a Streckeisen diagram (Streckeisen, 1976) and have arc-type geochemical signatures. The analyzed sample contains 23% actinolite after hornblende(?), 10% chlorite and 2% muscovite both derived from biotite, 1% titanite, and minor apatite, zircon, monazite(?), and opaque minerals.

1 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8. 2 Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3. 3 All UTM coordinates are in NAD 83, Zone 12.


Figure 1 - Location map showing lithotectonic domainal classification of northwestern Saskatchewan and 2005 map areas.

The Milliken Lake pluton was analyzed using the ID-TIMS technique. The majority of recovered zircons were colourless to tan fragments displaying mineral inclusions, fractures, and some alteration. The zircon contained moderate to low uranium (126 to 242 ppm) and high Th/U ratios (1.20 to 1.46), consistent with primary zircon crystallized from a mafic magma. The five multi-grain (6 to 12 grains each) zircon fractions were between 2.7% and 5.7% discordant and had consistent 207Pb/206Pb ages that varied between 2313 and 2318 Ma. The weighted average 207Pb/206Pb age for all five fractions of 2315.6 ±0.6 Ma (MSWD=12) is considered the current best estimate for the age of crystallization.

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Figure 2 - Simplified geological map of the Harper Lake area of western Zemlak Domain showing sampling sites of augen granite (sample 4705-0607) and porphyritic granodiorite (sample 4705-0556); modified from Ashton et al. (2005).

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Figure 3 - Simplified geological map of southwestern Beaverlodge Domain showing location of Milliken Lake pluton (sample 4703-0006).

3. Sample Descriptions and Results from SHRIMP Study

a) Zin Bay Coarse-grained Granite, Western Nolan Domain (Sample 4705-4144c) The Nolan Domain (Figure 1) is dominated by felsic plutonic rocks (e.g. Ashton et al., 2005). Five previously dated samples from the central part of the domain yielded 2.64 to 2.58 Ga U-Pb zircon crystallization ages (Van Schmus et al., 1986). During recent reconnaissance mapping (Ashton et al., 2005), a sample of pink, medium- to coarse-grained granite containing K-feldspar phenocrysts up to 2 cm in long dimension (similar to Figure 10 from Ashton et al., 2005) was collected from Zin Bay in northwestern Tazin Lake (Figure 4; UTM 599097 m E, 6638976 m N). The sample was collected from a unit that extends southward into the Zemlak Domain, where it has been overprinted by an east-west–oriented tectonic front (Ashton et al., 2005). The weakly deformed, non-magnetic sample contained about 5% biotite, 1% subhedral epidote, and traces of titanite, apatite, zircon, and allanite/monazite.

Most zircon grains separated from the sample were euhedral prisms displaying fine to broad oscillatory growth zoning typical of igneous zircon. A total of eighteen analyses revealed a single population, with a concordant age of 2598.3 ±5.9 Ma (MSWD=0.80, probability=0.70), which is considered the best estimate of the igneous crystallization age of the rock.

b) Mylonitized Pink Leucogranite, Nolan-Zemlak Domain Boundary (Sample 4705-0214b) A sample of salmon pink leucogranite was collected from eastern Laird Island in central Tazin Lake (Figure 4; UTM 612632 m E, 6628085 m N) at the Nolan-Zemlak domain boundary. The sample was mylonitic with ribboned quartz, beaded feldspar porphyroclasts, and about 4% chlorite along with traces of sericite, apatite, zircon, and opaque minerals (Figure 16 from Ashton et al., 2005). Due to its leucocratic nature and similarity to pink leucogranites elsewhere in the Zemlak Domain (e.g. Ashton et al., 2005), it was initially interpreted as a crustal melt emplaced during development of the tectonic front, and collected with the idea that it would date the onset of accretion related to a ‘Taltson Orogen’.

Zircons from the pink leucogranite were dominantly prismatic and of variably poor quality, with extensive regions of alteration primarily in the outer portion of grains. Cores showed weak oscillatory zoning typical of igneous zircon. Of the 25 analyses conducted, 19 form a tight cluster overlapping concordia with a weighted mean age of 2584.2 ±4.1 Ma (MSWD=0.69, probability=0.82), which is interpreted as the crystallization age of the rock. A subset of analyses having younger 207Pb/206Pb ages decreasing to 2433 Ma are thought to reflect an undefined Pb

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loss event probably resulting from the mylonitization, and were excluded from the age calculation. Based on its age and lithological similarities, the pink leucogranite at this outcrop has been re-interpreted as a mylonitized equivalent of the coarse-grained granite from Zin Bay.

c) Porphyritic Hornblende Granite, Western Zemlak Domain (Sample 4705-0556) A pink, fine- to medium-grained, hornblende granite containing 15% K-feldspar augen up to 2 cm long was sampled from the northern Harper Lake area (Figure 2; UTM 571233 m E, 6616214 m N) in order to establish the age of a widespread, highly magnetic zone dominating much of the western Zemlak Domain. The sample was the least deformed of a variably mylonitic unit and contained about 10% hornblende, 3% chlorite after biotite, and minor titanite, apatite, and magnetite.

Zircons separated from the hornblende granite were dominantly prismatic with diffuse internal growth zoning. Some grains exhibited partially recrystallized cores. A total of 33 analyses were carried out and two dominant age populations identified. Nineteen analyses defined a cluster of concordant ages with a weighted mean of 2330.3 ±7.3 Ma (MSWD=1.3, probability=0.16). A second group of ten analyses have a weighted mean age of 1925.3 ±9.6 Ma (MSWD=1.8, probability=0.066). These latter analyses typically had higher uranium contents than the older group and lower Th/U ratios (0.15 to 0.6 versus 0.75 to 1.5). Where both age groups were observed within a single zircon grain, the younger age tended to come from the outer portion of the grain, which had a more-diffuse, less-zoned appearance that may indicate recrystallization of pre-existing zircon. Therefore, the tentative interpretation is that this rock crystallized at 2330.3 ±7.3 Ma and was metamorphosed at 1925.3 ±9.6 Ma.

d) Ena Lake Granodiorite-Tonalite (Sample 4705-0007) The Ena Lake granodiorite-tonalite (formerly Ena Lake quartz diorite of Koster, 1965) was sampled from an island in northern Ena Lake (Figure 5; UTM 657087 m E, 6646893 m N) to provide a representative age for the highly magnetic, arc-type rocks comprising the central Ena Domain. The pink medium-grained sample contained 2% K-feldspar augen up to 3 cm in length, along with 10% biotite, 5% hornblende, and traces of titanite, epidote, apatite, zircon, and magnetite (Figure 24 of Ashton et al., 2005).

Zircon separated from the Ena Lake granodiorite-tonalite was dominantly prismatic, fractured and locally altered. Internal growth zoning was characterized by broad diffuse zones. A total of 22 zircon analyses yielded a weighted mean age of 2325.1 ±7.4 Ma (MSWD=0.52, probability=0.97), which is interpreted as the crystallization age of this sample. The zircons have moderate to low uranium contents (50 to 200 ppm) and Th/U ratios close to unity. A single grain yielded a different age of 1922 ±16 Ma (three replicate analyses, one spot; MSWD=0.51, probability=0.60). The portion of the grain yielding this younger age has a flat homogenous character in back-scattered electron imagery and likely represents a recrystallized portion of a grain. The very low Th/U ratio (<0.05) is typical of metamorphic or secondary zircon (Heaman et al., 1990), suggesting that the age documents a regional thermal event in the area.

4. Summary New U-Pb zircon results from the Tazin Lake area (NTS 74N) have identified more 2.64 to 2.58 Ga granitic plutonism in the Nolan Block beyond that previously recognized by Van Schmus et al. (1986). The new results show that these ca. 2.6 Ga granitic rocks extend southward into the Zemlak Domain, where ca. 2.71 Ga granitoid rocks are also found in the southwest. At least some of the highly magnetic granitoid rocks and derived orthogneisses in the Zemlak Domain, along with an intermediate pluton in the western Beaverlodge Domain were emplaced at 2330 to 2315 Ma, suggesting that they are related to the Arrowsmith Orogen (Berman et al., 2005) to the north. The two ca. 2.3 Ga magnetic granitoid rocks were subsequently metamorphosed at 1925 to 1922 Ma during an inferred ‘Taltson Orogen’.

5. Acknowledgements Two of the samples analyzed at the Geological Survey of Canada were funded through the auspices of the Western Churchill Metallogeny Project.


Figure 5 - Simplified geological map of the northeastern Zemlak Domain showing sampling site of Ena Lake granodiorite-tonalite (sample 4705-0007); modified from Ashton et al., 2005).

6. References Ashton, K.E., Card, C., and Modeland, S. (2005): Geological reconnaissance of the northern Tazin Lake map area

(NTS 74N), including parts of the Ena, Nolan, Zemlak, and Taltson domains, Rae Province; in Summary of Investigations 2005, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.2, CD-ROM, Paper A-1, 24p.

Ashton, K.E., Kraus, J., Hartlaub, R.P., and Morelli, R. (2000): Uranium City revisited: a new look at the rocks of the Beaverlodge Mining Camp; in Summary of Investigations 2000, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.2, p3-15.

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6654000N

4705-0007

Location of dated sample

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Berman, R.B., Sanborn-Barrie, M., Stern, R.A., and Carson, C.J. (2005): Tectonometamorphism at ca. 2.35 and 1.85 Ga in the Rae Domain, western Churchill Province, Nunavut, Canada: insights from structural, metamorphic and in situ geochronological analysis of the southwestern Committee Bay Belt; Can. Mineral., v43, p409-442.

Heaman, L.M., Bowins, R., and Crocket, J. (1990): The chemical composition of igneous zircon suites: implications for geochemical tracer studies; Geochim. Cosmochim. Acta, v54, p1597-1607.

Koster, F. (1965): The Geology of the Ena Lake Area (east half), Saskatchewan; Sask. Dep. Miner. Resour., Rep. 91, 31p.

Streckeisen, A. (1976): To each plutonic rock its proper name; Earth-Sci. Rev., v12, p1-33.

Van Schmus, W.R., Persons, S.S., Macdonald, R., and Sibbald, T.I.I. (1986): Preliminary results from U-Pb zircon geochronology of the Uranium City region, northwest Saskatchewan; in Summary of Investigations 1986, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 86-4, p108-111.

new u-pb zircon age dates from the tazin lake map area ... · quartz diorite fields on a...

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