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Geological Survey Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8
Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, D.C. 20015
Consulting Geologist, 509 Windermere Avenue, Ottawa, Ontario, Canada K2A 2W3
Corresponding author: E-mail, lhulbert{at}nrcan.gc.ca
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Abstract |
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 Top Abstract IntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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Re-Os isotope systematics of mineralized samples from seven deposits that span the length of the belt produce an age similar to the U-Pb zircon age but with a much larger error (1885 ± 49 Ma with an initial 
Os of 4.6). The Os isotope data demonstrate that the source of the Thompson belt ores, including those orebodies hosted in ultramafic rocks and those hosted in pelitic rocks, was dominantly mantle derived. Establishing that Thompson belt magmatism is synchronous with the Molson dike swarm and the extensive Fox River sill has important new exploration implications and requires modification of existing concepts which have presumed that Thompson belt magmatism is significantly older than these other intrusive suites. Also, it would appear that other important Ni and Ni-Cu deposits elsewhere in the Circum-Superior belt and in the Trans-Hudson orogen formed at or near 1.88 Ga, and thus this age could be considered an important and widespread nickel mineralizing metallogenic interval during the Early Proterozoic.
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Introduction |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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This paper presents the results of a U-Pb zircon study conducted on a mineralized ultramafic body from the Setting Lake area in the southern portion of the belt and an Re-Os isotope geochronologic investigation based on material from seven deposits which span the entire length of the exposed portion of the Thompson belt. The present Re-Os isotope study may be compared directly with several other relevant Ni and/or PGE sulfide investigations at Rankin Inlet, Canada (Hulbert and Grégoire, 1993), Pechenga Complex, Finland (Walker et al., 1997), Kambalda, Australia (Foster et al., 1996), Noril’sk, Russia (Walker et al., 1994), Voisey’s Bay, Canada (Lambert et al., 1999), and the Bushveld Complex, South Africa (Schoenberg et al., 1999). The results presented here document the first Re-Os isotope investigation conducted on mineralized ultramafic rocks and associated nickel sulfide ores from the Thompson nickel belt.
The significance of these age determinations relative to other precisely dated mafic and ultramafic bodies in the region and speculation on the exploration potential of other mafic-ultramafic bodies in the region are discussed.
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Regional Setting of the Thompson Nickel Belt and its Sulfide Deposits |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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The Early Proterozoic supracrustal rocks of the Superior boundary zone comprise part of the extensive Circum-Superior belt (Baragar and Scoates, 1981). The northern portion of the belt borders the Archean Superior craton and consists of a semicontinuous succession of supracrustal rocks defined by the Labrador trough (New Quebec orogen), the Cape Smith belt, the Ottawa Islands, the Belcher Islands, Richmond Gulf, the Sutton inlier, the Fox River belt, and the Thompson nickel belt (Fig. 2, inset). In Manitoba the Superior boundary zone has been divided into three main segments from west to east: the Thompson nickel belt, the Split Lake block, and the Fox River belt, respectively (Weber and Scoates, 1978; Fig. 2). Most of the Superior boundary zone (i.e., Thompson nickel belt, the Split Lake block, and the southern margin of the Fox River belt) consists of polymetamorphic migmatitic gneisses which are derived from Pikwitonei-type granulites through selective recrystallization and retrograde metamorphism, structural reconstitution, and migmatization under amphibolite-grade metamorphic conditions (Weber and Scoates, 1978; Böhm et al., 1999). Thermotectonic overprinting of rocks in the Superior boundary zone was related to collisional tectonics during the late compressional phase (1820–1720 Ma) of the development of the Trans-Hudson orogen (Machado, 1990; Machado et al., 1990).
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The Ospwagan Group and basement rocks of the Thompson belt have been intruded by granitoids that range in age from 1822 to 1726 Ma (Machado, 1990). The oldest collisional pluton in the belt is a 1836 Ma diorite gneiss (Bleeker and Macek, 1995). Granitic pegmatites were intruded between 1786 and 1760 Ma and are associated with retrograde amphibolite facies metamorphism and F2 to F3 deformation (Bleeker, 1990a, 1996; Machado, 1990).
Thompson belt supracrustal rocks have been correlated with those of the Fox River belt (Baragar and Scoates, 1981; Scoates, 1981) on the basis of lithological similarities in the supracrustal succession and similar chemical composition of the ultramafic and mafic volcanic and intrusive rocks. Bleeker and Macek (1988) suggested that metasedimentary rocks from the Pipe Lake open pit are older than Fox River belt supracrustal sequences because they have undergone a period of metamorphism and deformation prior to the intrusion of mafic dikes equated to the 1883 Ma Molson swarm. Weber (1990) noted that although the available radiometric data are not precise enough to establish definite age relationships between the two supracrustal sequences, they are consistent with the findings of Bleeker and Macek (1988). Machado et al. (1987b) obtained a U-Pb age of 1776 Ma on titanite from amphibolite interpreted to be a Molson dike in the Pipe open pit, providing a minimum age for emplacement of the unit and the supracrustal sequence that it intruded.
Ultramafic rocks in the Thompson nickel belt occur in a zone about 6 km wide along the western side of the belt in metasedimentary and metavolcanic rocks and gneisses, the largest bodies being confined to the metasedimentary rocks. The ultramafic bodies are tabular or lenslike and range from less than 1 m to approximately 1 km wide (Peredery et al., 1982). Contact relationships with the enclosing country rock are broadly conformable, although, locally, deformation and alteration may mask the original contact relationships. Many of the olivine-rich rocks (i.e., dunite and peridotite) have been converted to serpentinite. The extent of alteration and deformation commonly diminishes toward the center of the bodies. Where least deformed and altered, the ultramafic bodies appear to be sill-like and contain a lithological zonation consisting of a dunitic core surrounded by poikilitic harzburgite which grades toward the outer contacts into olivine-rich orthopyroxenite and orthopyroxenite. This lithological zonation is similar to that found in other ultramafic bodies elsewhere in the Circum-Superior belt, (e.g., in the Katiniq sill, Cape Smith belt, Quebec: Barnes et al., 1982). All ultramafic bodies in the Thompson belt possessing the dunite-peridotite (harzburgite) ± orthopyroxenite association are of komatiitic affinity (Naldrett et al., 1979; Theyer, 1980; Peredery et al., 1982; Good, 1985; Bleeker, 1990b, 1996; Good and Naldrett, 1993).
Nickel sulfide mineralization occurs in close association with the ultramafic bodies and in the adjacent country rocks. The sulfide mineralogy is simple and consists of pyrrhotite, pentlandite, minor chalcopyrite, and cubanite with traces of sphalerite (Peredery et al., 1982). Most of the nickel sulfide mineralization is variably metamorphosed and recrystallized and in places displays metamorphic banding. About 75 percent of all known nickel sulfide mineralization in the Thompson nickel belt is directly associated with ultramafic bodies; the remaining 25 percent is associated with metasedimentary rocks and gneisses (Peredery et al., 1982). Disseminated (including heavy disseminated and net-textured sulfides), massive and breccia ores represent the main types of sulfide mineralization in the Thompson belt. A review of these and other less common types of mineralization in the belt can be found in Peredery et al. (1982).
Middle to upper amphibolite facies metamorphism is pervasive throughout the Ospwagan Group, though areas of lower metamorphic grade (upper greenschist to lower amphibolite) are not uncommon. The ultramafic bodies appear to have survived the effects of metamorphism and deformation better than most other lithologic units. In a number of ultramafic bodies, it is apparent that the margins of the intrusions have taken up most of the deformation and metamorphic effects, whereas the interior portions are relatively well preserved.
Previous geochronology
Early K-Ar and Rb-Sr geochronologic studies of Thompson belt gneisses (Cranstone and Turek, 1976; Theyer, 1980; Brooks and Theyer, 1981) produced results suggesting protolith ages ranging from 2960 to 2830 Ma, with resetting between approximately 1800 and 1670 Ma, all with relatively large uncertainties (± 35–440 Ma). Similarly, Rb-Sr geochronologic studies of Ospwagan Group sedimentary rocks (Theyer, 1980) yielded imprecise ages ranging from 1880 to 1660 Ma (± 90–200 Ma).
Cumming et al. (1982) conducted a Pb isotope investigation of nickel sulfides from seven deposits of the Thompson nickel belt in an attempt to directly date Thompson belt nickel sulfides. Data were sufficiently well correlated from four deposits to define isochrons whose mean corresponds to an age of 2320 ± 30 Ma. This age was believed to represent the emplacement age of the ultramafic bodies and associated Ni ores. The same study revealed secondary Pb isochrons which were interpreted as recording a possible early folding event in the Thompson belt supracrustal rocks (2015 ± 15 Ma), probable late-stage Trans-Hudson retrogression (1620 ± 25 Ma), and thermal effects and intrusion of the Mackenzie dike swarm (1125 ± 60 Ma).
A recent detrital zircon study of Ospwagan Group sedimentary rocks by Bleeker and Hamilton (2001) has shown that both a basal orthoquartzite of the Manasan Formation and a stratigraphically higher metagraywacke of the Setting Formation reflect derivation principally from Pikwitonei-type (Archean, Superior province) basement (ages mostly ca. 2800–2600 Ma; dominant age mode between 2700–2675 Ma). However, the Setting Formation graywacke was also found to contain a single Paleoproterozoic-aged detrital grain with an interpreted crystallization age of 1974 ± 50 Ma (2
error; Bleeker and Hamilton, 2001). Deposition of at least the upper part of the Ospwagan Group is thereby constrained to be contemporaneous with or younger than a period of active magmatism at 1974 Ma.
Mafic to ultramafic dikes of the north-northeast–trending Molson swarm (Ermanovics and Fahrig, 1975; Scoates and Macek, 1978; Paktunç, 1987) intruded the Archean crust along the northwestern portion of the Superior craton (Fig. 2) and are considered to be comagmatic with the Fox River sill based on chemical similarities (Scoates and Macek, 1978) and age (Heaman et al., 1986). Two of the Molson dikes have been dated previously: the Cuthbert Lake dike, of mafic to ultramafic composition, has an age of 1883 ± 2 Ma, whereas the gabbroic Cross Lake dike yielded an age of 1883.7+1.7–1.5 Ma (Heaman et al., 1986). A U-Pb zircon age of 1882.9+1.5–1.4 Ma has been determined for the Fox River sill (Heaman et al., 1986). Bleeker and Hamilton (2001) provided SHRIMP U-Pb ages from a metagabbro dike that cuts the basal unconformity of the Ospwagan Group in the Thompson south pit, concluding that relict igneous zircons preserved magmatic ages of ca. 1855 Ma or slightly older (approaching Molson dike ages), whereas metamorphic zircon rims developed at ca. 1780 Ma. Recent geochronologic (Heaman and Corkery, 1996) and paleomagnetic (Zhai and Halls, 1994) investigations of mafic dikes in the Pikwitonei granulites and the Split Lake block (Fig. 2) indicate that at least two distinct generations of mafic dikes exist in the region, as exemplified by the northeast-trending Cauchon dike (2092 Ma: Heaman and Corkery, 1996) and the Birthday Rapids dike (2070 Ma: Heaman and Corkery, 1996). The older ages are in part the basis for an inferred ca. 2100 Ma age of rifting of the Superior craton (Zhai and Halls, 1994; Bleeker, 1996; Heaman and Corkery, 1996), whereas the younger age (1883 Ma) is believed to represent a possible marginal basin rifting event that postdated F1 deformation in the Thompson belt (Bleeker, 1990a). This latter event may have been associated with the early evolution of the Reindeer zone of the Trans-Hudson orogen (Lucas et al., 1996).
The western limit of the Thompson nickel belt is in sharp contact with the Kisseynew gneiss domain of the Reindeer zone. Locally this is demarcated by the Setting Lake fault which occurs along the west shore of Setting Lake. Geochronologic investigations along this zone indicate that the oldest metamorphism occured at 1786 ± 3 Ma and that this fault zone may have been active as late as 1768 Ma (Machado and David, 1992). At Ospwagan Lake, a granitic pegmatite which intruded Ospwagan Group rocks has been dated at 1772 ± 2 Ma (U-Pb monazite age: Machado and David, 1992) and indicates that pegmatite of this age cutting Thompson belt gneiss could be related to late movements along the Setting Lake fault.
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U-Pb Dating of Zircon from the Setting Lake Ultramafic Body |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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Although the three fractions B2, B3, and C3 yield the most consistent and older 207Pb/206Pb isotope ages of the entire dataset, their analyses are not completely overlapping, and only two of these plot within error of concordia. A calculated mean 207Pb/206Pb age of all three older fractions is 1880 ± 2 Ma, which, although likely an accurate reflection of the true crystallization age, probably underestimates the actual errors inherent in the spread of data. Rather, given the range of measured Pb/U ratios in this sample, we consider the age and uncertainty associated with the single, older concordant analysis (C3) to best represent a reasonable and conservative estimate of the magmatic age of the pyroxenite, at 1880 ± 5 Ma. The fractions with the lowest Pb/U ratios (A1, B1, C1) appear to define a discordia line whose lower intercept age is poorly approximated at ca. 1700 Ma (reference chord in Fig. 6), essentially indistinguishable from the estimated age of latest Trans-Hudson orogenic events in the Thompson belt as defined by Machado (1990). Fractions A2 and A3 fall below this discordia and may be a result of ancient and/or recent Pb loss. Table 1 shows that zircon fractions with the oldest 207Pb/206Pb ages consistently have model Th/U in the range of 0.68 to 0.86, similar to or approaching values observed in mafic rocks (Heaman et al., 1990; Paces and Miller, 1993; Hamilton et al., 1998). In contrast, Th/U ratios correlate directly with decreasing 207Pb/206Pb ages for fractions A1, C1, and B1, falling from 0.57 to 0.33, suggesting that these analyses represent mixtures containing low Th/U metamorphic zircon overgrowths that were not completely removed by preanalysis air abrasion treatment. These observations are entirely consistent with the SHRIMP results obtained by Bleeker and Hamilton (2001) from analogous igneous and metamorphic zircon domains in metagabbro from the Thompson south pit, where emplacement occurred at or slightly before ca. 1855 Ma and was followed by development of low Th/U metamorphic rims on the zircons at ca. 1780 Ma.
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Re-Os Dating of Mineralized Ultramafic Rocks and Nickel Ores, Thompson Nickel Belt |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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) and an initial 187Os/188Os ratio of 0.1222 ± 0.0039 at the 95 percent confidence level corresponding to Os = 7.0 ± 3.4 using a model 3 fit from Isoplot (Ludwig, 1999). The Os units are the percentage deviation of an Os isotope composition at any time from that of the chondritic average. Omission of data for three samples from the regression allows the age to be more precisely defined. These samples include: two harzburgites with disseminated sulfides (from Moak Lake, 12912–430, and from the Setting Lake deposit, 74276–686), and duplicate analyses of a sulfide segregation in a harzburgite (from Thompson 74381–2152.2), all of which lie above the isochron. If these four data points are excluded, a regression of the remaining data yields an Re-Os isochron with an age of 1885 ± 49 Ma (2), an initial ratio of 0.1195 ± 0.0026 (95% confidence level), and Os of 4.6 ± 2.3. The mean square weighted deviates (MSWD) of 80 for this regression (using the 2 errors given in Table 2) indicate that the scatter about the isochron exceeds the analytical uncertainties and probably results from small variations in the initial ratios that are present even in the filtered data. Contamination by a few percent of an 187Os-rich component could have shifted the composition of these samples to values above the Re-Os isochron. Such a component may have been added by small amounts of crustal contamination prior to emplacement or by later minor addition from metamorphic fluids.
Although the Re-Os isochron age for Thompson belt mineralized ultramafic rocks appears to be indistinguishable from the zircon age obtained from the south Setting Lake ultramafic body, the large error implies that the age is much more uncertain. The similarity in Os initial ratios between massive sulfides and ultramafic rocks indicates that Os in the sulfides was derived from the same source as that of the ultramafic rocks. Moreover, in the Birchtree deposit, values for Os in pelite-hosted sulfide ores (90768–104.2, 88103–103) are similar to those for peridotite-hosted sulfide ores (89669–89667, 89669–89647, 89669–89666.5, 89669–89665). This suggests that the Os in the pelite-hosted Ni sulfides and the magmatic Ni sulfides had a common source.
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Os value of the isochron of 4.6 ± 2.3 reflects a time-averaged Re/Os ratio that is resolvably higher than chondritic, which could result from a small amount of pervasive crustal contamination or from derivation from a mantle source that had a history of suprachondritic Re/Os. Whereas a detailed evaluation of the Os isotope history of the source for the Thompson belt is beyond the scope of this paper, it may be difficult to retain an ultramafic melt composition during crustal contamination (e.g., Puchtel et al., 2001a). Figure 8 compares the Os isotope composition of the Thompson belt with age in a variety of mafic-ultramafic systems that have been interpreted to have been derived dominantly from mantle plumes. The values measured for the Thompson belt are well within the range measured for other Proterozoic systems and suggest that an 187Os-enriched mantle reservoir had already formed by this time (e.g., Walker and Nisbet, 2002). The age of this magmatic episode equates with the proposed mantle superplume event at 1.9 Ga (Condie et al., 2000). |
Discussion and Conclusions |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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A mafic-ultramafic sill with associated Cu-Ni sulfide mineralization from the Labrador trough (New Quebec orogen) has a U-Pb age of 1883.8 ± 1.5 Ma (T. Birkett et al., in Wardle et al., 1990). This is identical with ages of Thompson belt mineralized ultramafic rocks, Molson dikes, and the Fox River sill and supports the suggestion that magmatism along the northwestern margin of the Superior craton may have been mirrored by contemporaneous magmatism along its eastern margin (e.g., Scoates, 1981; Heaman et al., 1986).
The results of this combined U-Pb zircon and Re-Os isotope investigation have demonstrated that crystallization of Thompson belt ultramafic rocks and ores were contemporaneous with the Fox River sill and at least two dikes of the Molson swarm. These results imply that mafic-ultramafic igneous activity occurred over a large portion of the northwest Superior craton and its boundary zone during a narrow interval of time at ca.1.88 Ga. In the Thompson belt, Os isotope data show that Ni sulfide mineralization was contemporaneous with ultramafic igneous activity, and that the source of the ores has the same Os isotope composition as the associated ultramafic rocks.
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APPENDIX 1. Location and Geology of the South Setting Lake Ultramafic Intrusion |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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The south Setting Lake ultramafic body occurs near the southern end of the east arm of Setting Lake and is approximately 1,400 m long and 90 to 190 m wide. It is fault bounded to the south (Fig. 3) and confined to a metasedimentary (semipelite) and metavolcanic (amphibolite) sequence that also contains thin horizons of dolomitic marble (skarn), sulfide and silicate facies iron formation, and graphite-rich pelitic schists. A steeply dipping fault runs along the length of the east arm of the lake and separates the ultramafic body and the bulk of the enclosing metasedimentary and metavolcanic rocks from migmatitic Archean basement (Fig. 3). The contorted nature of the ultramafic body suggests that the body has been folded. Semimassive sulfide accumulations are present along the western contact of the body.
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Medium- to coarse-grained pyroxenite is variable in texture, and pegmatitic varieties have bronzite crystals, 1.5 to 2.0 cm in diameter. Pegmatitic pyroxenite consists mainly of orthopyroxene, plagioclase, phlogopite, and interstitial sulfide. Although these rocks retain their magmatic texture, and macroscopically appear relatively fresh, thin section examination reveals that they have been subjected to partial serpentinization and regional metamorphism. Nevertheless, fresh relict olivine, orthopyroxene, and chromite are present. A representative chemical analysis of this material and the material sampled for U-Pb zircon geochronology is provided in Table A1, where its ultrabasic character is clearly demonstrated. The presence of sulfides in this sample is reflected in the elevated levels of nickel (6,400 ppm), copper (1,500 ppm), and sulfur (2.71 wt %).
The normative mineralogy (wt %) for sample W106-06 is 47.8 percent orthopyroxene, 20.8 percent olivine, 17.8 percent plagioclase, 2.5 percent Fe-Ti oxides, 1.5 percent chromite, and 9.5 percent sulfides. The Mg number (i.e., Mg/Mg + Fe2+) of the rock is 0.875, and the calculated Mg numbers for normative orthopyroxene and olivine are 0.875 and 0.853, respectively. The normative anorthite content is 90.3 percent.
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APPENDIX 2. Sample Selection |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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For the Re-Os study, samples were selected to represent Ni sulfide mineralization from throughout the belt (Fig. 1). Those ore deposits sampled included Moak Lake, Thompson, Birchtree, Airport, Pipe 2, Setting Lake, and Bucko. Only mineralized ultramafic samples, demonstrating well-preserved magmatic sulfide-silicate textures and low degrees of alteration, were included in the study. Massive sulfides chosen from within or in contact with ultramafic bodies also satisfied these criteria. Pelite-hosted massive sulfides were included if they had high Ni, Cr, and PGE concentrations. All samples had a mass of at least 1 kg.
Samples from the Thompson mine are from the 1-D orebody (74381, 74379, 74391) and the south pit (WY87-9a). The samples represent massive, semimassive, and disseminated sulfides. Geological and textural relationships indicate that the sulfides are all of magmatic origin. In the Birchtree mine, massive magmatic sulfides (89669-67, 89669-47, 89669-6.5), semimassive, net-textured Cu-rich sulfides (89669-5), and high-grade massive sulfides with pelite inclusions (90768-104.2, 88103-103) were investigated. All samples except 88103-103 are associated with harzburgite. The folded Setting Lake deposit contains massive sulfides (containing between 6.4 and 7.3% Ni) in dunite (74285-1008.2-.6) and harzburgite (74276-480.5), and harzburgite-hosted disseminated sulfides (74276-478.5, 74276-686). The Pipe 2 deposit contains massive sulfide with and without ultramafic inclusions (86237-5353.5). The Airport deposit (86251-1800) and the Moak deposit (12912-430) contain disseminated and massive sulfides. The Bucko deposit contains disseminated sulfides in harzburgite and/or dunite and the tenor of the metals in this deposit is higher than that of other Thompson belt deposits (Good, 1985; Good and Naldrett, 1993).
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APPENDIX 3. Analytical Methods |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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). Average Pb and U blanks during the course of analysis were 2 and <0.5 pg, respectively.
Re-Os isotope geochemistry
The techniques for chemical separation of Re and Os have been published previously (Horan et al., 1995, and references therein). Samples were processed by alkaline fusion (Morgan and Walker, 1989) and/or by aqua regia digestion in sealed Pyrex tubes (Shirey and Walker, 1995) using 200 to 400 mg of sample that had been spiked with 190Os and 185Re. Osmium was separated by double distillation from H2SO4-H2O2 followed by anion exchange, and Re was purified by anion exchange. For both dissolution methods, total analytical blanks were 40 pg for Re and 3 pg for Os. All data were blank corrected and the uncertainties incorporate the uncertainties in the blank. The isotopic composition of Re and Os were determined by negative thermal ionization at the University of Maryland at College Park, using a VG Sector 54 mass spectrometer (Walker et al., 1994), and at the U.S. Geological Survey, using an NBS design 12" mass spectrometer (Horan et al., 1995).
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Acknowledgments |
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D. Peck, P. Golightly, M. Lesher, J. Robertson, and J. der Weduwen critically reviewed an early version of the manuscript. The comments and suggestions of these people and Economic Geology reviewers have helped to improve the manuscript and are greatly appreciated.
January 3, 2003; December 1, 2004
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Footnotes |
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January 3, 2003; December 1, 2004
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References |
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TopAbstractIntroductionRegional Setting of the...U-Pb Dating of Zircon...Re-Os Dating of Mineralized...Discussion and ConclusionsAPPENDIX 1. Location and...APPENDIX 2. Sample SelectionAPPENDIX 3. Analytical MethodsReferences |
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——1987, Volcanic geochemistry of the northern segments of the Circum-Superior belt of the Canadian Shield: Geological Society Special Publication 33, p. 113–131.
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Bell, C.K., 1971, Boundary geology, upper Nelson River area, Manitoba and northwestern Ontario: Geological Association of Canada Special Paper 9, p. 11–39.
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——1990b, Thompson area—general geology and ore deposits: Geological Survey of Canada Open File 2165, p. 93–136.
——1996, The Thompson nickel belt and its nickel deposits: A review [abs.]: Geological Association of Canada Program with Abstracts, v. 21, p A9.
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Bleeker, W., and Macek, J.J., 1988, Thompson nickel belt project—Pipe pit mine: Manitoba Energy and Mines Report of Field Activities 1988, p. 111–115.
——1996, Evolution of the Thompson nickel belt, Manitoba: Setting of Ni-Cu deposits in the western part of the Circum-Superior boundary zone (Field Trip A1): Geological Association of Canada-Mineralogical Association of Canada Annual Meeting Field Trip Guidebook, 44 p.
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Chandler, F.W., and Parrish, R.R., 1989, Age of the Richmond Gulf Group and implications for rifting in the Trans-Hudson orogen, Canada: Precambrian Research, v. 44, p. 277–288.[CrossRef][Web of Science][GeoRef]
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Cranstone, D.A., and Turek, A., 1976, Geological and geochronological relations of the Thompson nickel belt, Manitoba: Canadian Journal of Earth Sciences, v. 13, p. 1058–1069.[GeoRef]
Cumming, G.L., Eckstrand, O.R., and Peredery, W.V., 1982, Geochronological interpretations of Pb isotope ratios in nickel sulphides of the Thompson belt, Manitoba: Canadian Journal of Earth Sciences, v. 19, p. 2306–2324.[GeoRef]
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