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U-Pb Zircon and Re-Os Isotope Geochronology of Mineralized Ultramafic Intrusions and Associated Nickel Ores from the Thompson Nickel Belt, Manitoba, Canada

L. J. Hulbert and M. A. Hamilton^*

Geological Survey Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8

M. F. Horan

Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, D.C. 20015

R. F. J. Scoates

Consulting Geologist, 509 Windermere Avenue, Ottawa, Ontario, Canada K2A 2W3

View larger version (36K): [in a new window]   FIG. 1. Simplified geologic map of the Thompson nickel belt (TNB) segment of the Superior boundary zone, showing the adjacent Reindeer zone of the Trans-Hudson orogen (Churchill province), the Superior province, and the northern edge of the Paleozoic cover. Within the TNB the names and locations of all known significant Ni-Cu deposits and properties are shown, as well as the locations of known serpentinized ultramafic bodies and early Proterozoic supracrustal sequences. The location of the U-Pb zircon sample from the southern end of the eastern arm of Setting Lake is indicated by the label W106-6.

View larger version (64K): [in a new window]   FIG. 2. Generalized geologic map, showing the northwestern Superior province (SP), northeastern Trans-Hudson orogen (THO) and Churchill province (CH), Phanerozoic cover (PH), Thompson nickel belt (TNB), Fox River belt (FRB), Split Lake block (SLB), and Split Lake fault (SLF) segments of the Superior boundary zone (shaded), Pikwitonei granulite domain (PGD), dikes related to the Molson swarm (Cross Lake, CR; Cuthbert Lake, CT; Nelson River, NR), and the location of U-Pb zircon sampling sites from this study (W106-6) and Heaman et al. (1986; MAN 85-32, 311, 110, 201-6). Geology reproduced in part from figure 1 of Heaman et al., (1986) and the Geological Map of Manitoba (Manitoba Mineral Resources Division, 1979). The inset shows the distribution of Proterozoic supracrustal rocks (black) of the Circum-Superior belt (CS, Cape Smith belt; OI, Ottawa Islands; BI, Belcher Islands; RG; Richmond Gulf; SI, Sutton inlier; NQO, New Quebec orogen, formerly known as Labrador trough, LT; GP, Grenville province) and designations mentioned above.

View larger version (76K): [in a new window]   FIG. 3. Inferred geology of the U-Pb zircon sample site (W106-6) locality, southern end of the eastern arm of Setting Lake, based on Preliminary Map 2001 FN-2 (Thompson Nickel Belt Working Group, 2001), Falconbridge geophysical surveys, diamond drill logs, and surface mapping in the area. Note that most of this area is covered by Setting Lake and surface exposure is limited.

View larger version (42K): [in a new window]   FIG. 4. Simplified graphic log of the geology encountered in diamond drill hole 106-6 with an expanded lithologic representation of the ultramafic intrusion, location of material sampled for the U-Pb zircon study, and associated magmatic Ni-Cu sulfide mineralization.

View larger version (47K): [in a new window]   FIG. 5. Representative backscattered electron (BSE; left) and cathodoluminescence (CL; right) image pairs for zircons from orthopyroxenite sample W106-6. Note preservation of delicate, regular, straight igneous zoning in image (b), whereas others show sharp, fine to broad oscillatory and sector magmatic zoning, as in (a), (c), (d), and (e). All zircons in images (a) through (e) also show evidence of thin, irregular overgrowths or rims lacking significant zoning.

View larger version (24K): [in a new window]   FIG. 6. Concordia diagram showing U-Pb data for zircons from the south Setting Lake ultramafic intrusion. The error ellipses reflect two sigma errors and the corresponding alphanumeric designations refer to fractions listed in Table 2. Reference chord (dashed line) is shown, anchored at 1880 Ma. See text for discussion.

View larger version (7K): [in a new window]   FIG. 7. 187Re/188Os vs. 187Os/188Os isochron diagram for samples from the Thompson nickel belt. The line represents model 3 least squares regression (Ludwig, 1999) of those samples (n = 23) shown with solid symbols. Errors for the age and initial 187Os/188Os are given at the 95 percent confidence level. The MSWD of this regression line is 80, using the 2 errors given in Table 3. Four data shown in open symbols were omitted from the regression line and are discussed in the text.

View larger version (9K): [in a new window]   FIG. 8. Comparison of initial Os in the Thompson nickel belt (TNB) with mafic and ultramafic systems that are interpreted to have been derived dominantly from mantle plumes. The Os units are the percentage deviation of an Os isotope composition at any time from that of the chondritic average. The horizontal black line represents the evolution of the chondritic average. Solid points are Os initial ratios calculated from isochrons. The boxes show the range of compositions for isotopically heterogeneous systems. Data sources are as follows: Bilingwe, Zimbabwe (Walker and Nisbet, 2002); Noril’sk, Siberia flood basalt province (Horan et al., 1995); Keweenawan (Shirey, 1997); Deccan, India (Allègre et al., 1999); Pechenga, Russia (Walker et al., 1997); Onega, Russia (Puchtel et al., 1999); Vetreny, Russia (Puchtel et al., 2001b); Alexo, Canada (Gangophadyay and Walker, 2003); Ruth’s Well, Australia (Meisel et al., 2001); Kambalda, Australia (Foster et al., 1996); Boston Creek, Canada (Walker and Stone, 2001); Kostomuksha, Russia (Puchtel et al., 2001b); Gorgona Island, Colombia (Walker et al., 1991); Viet-Song La, Vietnam (Hanski et al., 2004). Shown for comparison is the range of data defined by modern ocean island basalt (OIB; Shirey and Walker, 1988).