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Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 2

Basement Lithologic Framework and Structural Features of the Western Athabasca Basin

C.D. Card, J.E. Campbell, and W.L. Slimmon

Card, C.D., Campbell, J.E., and Slimmon, W.L. (2003): Basement lithologic framework and structural features of the western Athabasca Basin; in Summary of Investigations 2003, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003-4.2, CD-ROM, Paper D-3, 17p.

Abstract The final phase of field work of the Western Athabasca Basement Project included core logging in the Hook Lake and Carswell areas. The Careen Lake Group supracrustal package at Hook Lake is dominated by psammites and quartzites with subordinate psammopelites and migmatitic to diatexitic pelites. It is not known whether rare mafic rocks are part of, or intrude, the Careen Lake Group. All these are cut by peraluminous granites. In contrast, the Careen Lake Group in the Carswell Structure is dominated by psammopelites and migmatitic to diatexitic pelites, with subordinate psammites and quartzites. It is intruded by granodiorites to quartz diorites believed to be related to the ca. 1.986 to 1.960 Ga calc-alkaline intrusions of the Taltson Magmatic Zone and younger granites.

The Careen Lake Group has been metamorphosed under upper amphibolite to granulite facies conditions and folded at least twice. On the other hand, the later granodiorites and quartz diorites appear to be more weakly deformed and less metamorphosed than the Careen Lake Group. This may be due to differences in the rheological properties of the rocks. It is certain that all of these rocks were metamorphosed at ca. 1.94 to 1.90 Ga, an event that led to the generation of many of the younger, anatectic granites. Ductile structures formed during the Taltson-Thelon and Trans-Hudson orogenies were repeatedly reactivated by post-Trans-Hudson faulting. Near Hook Lake, a normal fault superposed on an older ductile structure created a half graben that influenced deposition of the lower Athabasca Group.

A number of findings have resulted from this project. The oldest rocks recognized are the Careen Lake Group, which may be Archean. Alternatively, ca. 2.5 Ga granitic gneisses of the Clearwater Domain may form their basement. A ca. 1.985 to 1.968 Ga intrusive suite dominated by granodiorites and quartz diorites provides a minimum age for the Careen Lake Group and is considered the equivalent of 1.986 to 1.960 Ga intrusive rocks in the Taltson Magmatic Zone. These rocks were subjected to 1.94 to 1.90 Ga high-grade metamorphism during which a suite of dominantly peraluminous granites was emplaced. Trans-Hudson granites are also present in the region but have so far only been identified near the Snowbird Tectonic Zone and in the Clearwater Domain, indicating that their emplacement was structurally controlled.

Older ductile structures such as the Snowbird Tectonic Zone have been repeatedly reactivated. Preliminary analysis of Landsat and Shuttle Radar Topography Mission digital elevation models suggests that most of the structures that cut the Athabasca Group and ultimately played a role in the formation of present-day landforms are likely related to two major regional fault systems, the Snowbird Tectonic Zone and the Tabbernor Fault system. This analysis also allows some time constraints to be placed on the age of regional fault reactivation, including displacement that occurred after formation of the Ordovician Carswell Structure.

Keywords: Rae Province, Western Athabasca Basin, Lloyd Domain, Clearwater Domain, Taltson Magmatic Zone, Hook Lake, Carswell Structure, Careen Lake Group, fault reactivation, Landsat and SRTM DEM, Snowbird Tectonic Zone, Tabbernor Fault system.

1. Introduction Although the EXTECH IV Athabasca Uranium project is nearly complete, the Western Basement Project is continuing. This year’s activities included investigation of core from the Hook Lake region and the northern part of the Carswell Structure as well as examination of cores from the Clearwater Domain. This work finishes the field-based component of the project, fills a major data gap and, when considered with geochronological, structural and metamorphic data, allows introduction of a viable, albeit incomplete, regional geologic framework. Field investigation of Carswell core was carried out at the Cluff Lake uranium mine, where production has ended and site decommissioning work is beginning. Work at Hook Lake was conducted from a small bush camp serviced by float planes from Voyage Air in Buffalo Narrows.


In this report we present the findings of investigations of core at the locations outlined above, a review of the findings of this project, and preliminary interpretation based on satellite/radar topography images, of the late structural features that affected the western Athabasca Basin. This work provides better time constraints on fault reactivation in the basin and place the faults within a regional framework.

2. Regional Geology The basement to the southern part of the Athabasca Basin west of the Snowbird Tectonic Zone (Figure1) mainly lies within the Lloyd Domain of the Rae Province. Exposure of the Lloyd Domain is restricted to the area south of the Athabasca Basin, but it can be traced northwestward beneath the Athabasca Group and Phanerozoic cover as a distinctive set of alternating high and low aeromagnetic lineaments (Geological Survey of Canada, 1987) to the western margin of the Athabasca Basin in northeastern Alberta, where they are overprinted by the Taltson aeromagnetic high (Figure 2). The Lloyd Domain aeromagnetic pattern is overprinted by the Clearwater aeromagnetic high (Figure 2), which is underlain by granitoid rocks of the Clearwater Domain and divides the Lloyd Domain into eastern and western segments (Figure 1; Card, 2002). The Lloyd Domain comprises a supracrustal package of unknown age called the Careen Lake Group (Scott, 1985), and two suites of intrusive rocks that are the equivalent of rocks exposed in the Taltson Magmatic Zone. In the Clearwater Domain, granitic gneisses are intruded by nearly undeformed granites.

3. Unit Descriptions: Hook Lake Lloyd Domain rocks have been previously described from core from the Carswell Structure and the area to the immediate west, south, and southwest, and in outcrop in the Careen Lake area in the east (Figure 1; Card, 2001,

2002). Cores from the Hook Lake area were examined this summer to bridge the gap between these southeastern and western extremities of the Lloyd Domain. As elsewhere in the Lloyd Domain, the basement rocks near Hook Lake comprise a supracrustal package, inferred to be a northwestward extension of the Careen Lake Group, and younger intrusive rocks dominated by peraluminous granites, but devoid of the quartz dioritic rocks of the southeastern Lloyd Domain. In contrast to other locales in the Lloyd Domain, where the supracrustal rocks are dominated by psammopelite and migmatitic to diatexitic psammopelite to pelite (Card, 2002), quartzite and psammite predominate at Hook Lake.

Psammite and psammopelite are the most common rock types observed in HK-series cores. The two lithologies are commonly interlayered, suggesting transposed primary sedimentary layering (Figure 3). Psammite is more abundant than psammopelite. Fresh examples are generally light grey and fine to medium grained, with 3 to 5% biotite and 1% garnet porphyroblasts. Intensive alteration generally resulted in

Figure 1 - Map of geological domains in Saskatchewan and northeastern Alberta, including the approximate location of the largely unexposed Clearwater Domain and the major structural features. Area 1, Hook Lake; Area 2, Northern Carswell Structure; LD, Lower Deck; and UP, Upper Deck.


replacement of biotite by chlorite and garnet by biotite and/or chlorite. Psammopelite is grey, contains 0.5 to 3 mm grains and 5 to 10% biotite and 3 to 5% garnet, both of which are commonly altered to chlorite and biotite and/or chlorite respectively. Rarely, up to 1% chalcopyrite is present. Both psammite and psammopelite are well foliated, commonly contain two foliations, and have been subjected to relatively high-grade metamorphic conditions. Both contain blue quartz, which can be indicative of granulite facies metamorphism (e.g. Niggli and Thompson, 1979) and psammopelitic rocks show pronounced leucosome-melanosome relationships in rare cases. Preferential melting of some psammopelitic layers is likely indicative of subtle compositional layering within these intervals and is further evidence for transposed primary sedimentary layering.

Figure 2 - Total field aeromagnetic map of northern Saskatchewan and northeastern Alberta showing the major structural features and aeromagnetic highs (Geological Survey of Canada, 1987).

Figure 3 - Interlayered psammite (light) and psammopelite (dark) from HK-6 (Table 1). Garnet is weakly altered in this example and not replaced by biotite and/or chlorite.


Intercalation of quartzite with psammite likely represents transposed primary bedding. Quartzite is generally white, medium grained, and contains 1 to 2% altered garnet and 1 to 4% variably altered biotite. Quartzite is distinguished from late quartz veins by its characteristic internal foliation. Although the quartzites may be older quartz veins, their spatial association with other metasedimentary rocks suggests a sedimentary protolith. Locally mylonitized quartzite exhibits ribboned quartz with at least 6:1 aspect ratios.

Rare pelite is generally restricted to sheared and/or faulted intervals indicating that it was a strain locus. Pelite has a matrix containing 0.5 to 3 mm grains of quartz, feldspar, 15 to 20% chloritized biotite, and 3 to 5% graphite with 2 to 3% chloritized garnet porphyroblasts up to 10 mm in diameter. Where associated with shear zones and faults, pelite is strongly foliated or brecciated and contains up to 2% sulphides found in carbonate veins within fault zones, including pyrite, chalcopyrite, chalcocite, pyrrhotite and bornite.

Nearly 20 m of green bleached rock in HK-3 (Table 1) may represent an amphibolite or mafic granulite (Figure 4). This rock ranges from medium to coarse grained and contains 40% of a fibrous, light green mineral interpreted to be a bleached amphibole, most likely hornblende, along with 10 to 20% chlorite and plagioclase. The strong association of this lithology with dolomitic veins in HK-3 suggests interaction between CO2-rich fluids and Ca-rich minerals, such as hornblende. A pervasive foliation, including major strain localization along both contacts likely formed under both brittle and ductile regimes, precludes this mafic rock from being a Mackenzie dyke, and hence makes it part of the Athabasca basement sequence. The age relationship of this rock with respect to the package of metasedimentary rocks described above or the younger granitoids described below is unclear, in part because of the high strain along the contacts. The upper 1 to 2 m of the interval displays 0.5 to 2 mm grains in contrast to most of the interior, which contains 3 to 5 mm grains, suggesting a chilled margin. Other more fine-grained layers are also common throughout the interval, but are notably absent at the bottom. This layering is interpreted as a relict primary feature. Based on the dominant 3 to 5 mm grain size, this rock is best interpreted as a layered intrusion of unknown age; however, the layering may also be a volcanic feature with the formerly fine-grained rocks coarsened during subsequent high-grade metamorphism, a common phenomenon in the region (Ashton and Card, 1998; Card, 2002).

Foliated peraluminous granite (Figure 5) is the most common intrusive rock in HK-series core and exhibits a strong foliation that is rarely mylonitic. These medium-grained granites vary in colour from pink to bleached white, although neither colour is likely indicative of fresh rock. Garnet porphyroblasts up to 10 mm in diameter and constituting 2 to 5% of the rock are commonly replaced by a combination of biotite and chlorite and the matrix contains 2 to 5% biotite or chlorite.

Medium-grained to pegmatitic massive peraluminous granite (Figure 5) lacks a strong foliation although feldspar laths are locally weakly aligned. It contains 2 to 4% biotite and 0 to 3% variably altered garnet with

Table 1 - Locations of diamond drill holes referenced in the text.

Drill Hole UTM E UTM N HK-1 624514 6402853 HK-2 619121 6404894 HK-3 619017 6407115 HK-6 634709 6403916 HK-7 631055 6398142 CAR-60 589397 6494802 BAN-8 580713 6484130 BAN-9 581116 6484716 SYL-1 566716 6470859

Figure 4 - Strongly altered mafic rocks in HK-3 (Table 1). Light spots are altered amphiboles. Note the common carbonate veins.

Figure 5 - Medium-grained, foliated peraluminous granite (top) and coarse-grained massive peraluminous granite (bottom). Dark spots are a combination of chloritized biotite and garnet. Drill hole is confidential.


rare occurrences of intergrown quartz and radiating tourmaline. Massive and foliated peraluminous granites are commonly spatially associated but their age relationships are unclear. Generally diffuse contacts suggest that they are broadly co-magmatic. The differences in deformation state may have resulted from contrasting rock strength properties.

4. Unit Descriptions: Carswell Structure A supracrustal package that included migmatitic to diatexitic psammopelite to pelite, quartzite, garnet-rich silicate facies iron formation and mafic granulite was observed in diamond drill-hole CAR-60, located about 2 km to the north of Carswell Lake (Card, 2001). These rock types are similar to those of the Murmac Bay Group exposed north of the Athabasca Basin in the Uranium City area (Hartlaub et al., in press) and it is tempting to suggest a correlation. Most of these lithologies are present elsewhere in the basement, but they are generally not closely spatial associated, nor is iron formation present. Several BAN- and CAR-series diamond drill holes from the north part of the Carswell Structure were examined in 2003 to determine if this association is a common feature of the area. The unit descriptions below show that in contrast to associations present in CAR-60 (Table 1), these cores contain the psammopelites and migmatitic to diatexitic pelites of the Careen Lake Group, along with intrusive granodiorites and peraluminous granites.

Grey psammopelite is the most common metasedimentary rock in this area. Psammopelite commonly has a 0.5 to 3 mm grain-size quartzofeldspathic matrix and contains 5 to 15% biotite, 1% graphite and 1% pyrite, and 2 to 5% variably altered garnet porphyroblasts up to 10 mm in diameter. Intervals of psammopelite are well foliated and display subtle transposed compositional layering indicative of primary layering.

Migmatitic to diatexitic pelite, a common lithology in the Carswell basement core and part of the Peter River gneiss (Tona et al., 1985), is interlayered with psammopelite. It is grey to dark grey and displays well-developed leucosome-melanosome relationships which define a migmatitic gneissosity (Figure 6). The medium-grained leucosome contains 5 to 10% garnet porphyroblasts up to 10 mm in diameter, whereas the fine-grained melanosome appears to comprise exclusively biotite.

Psammite and quartzite make up far less of the supracrustal package than at Hook Lake and are generally found as centimetre-scale layers within the psammopelites. Psammite is foliated, 0.5 to 2 mm in grain size and contains 1 to 3% garnet, 5% variably chloritized biotite, and up to 5% magnetite. Quartzites also contain a recognizable foliation, generally have 1 to 2 mm grain size, and contain 1 to 2% biotite.

Two intrusive suites are recognized in the north part of the Carswell Structure. The earliest contains granodiorite to quartz diorite and granitic gneiss. The granodiorites and quartz diorites are equigranular, 1 to 3 mm in grain size, and remarkably homogeneous (Figure 7). They contain 15 to 30% biotite ± hornblende and in contrast to most rocks in the area, they contain an L>S fabric. Other granodioritic to quartz dioritic rocks with such fabrics are found in SYL-1 (Table 1) and in Alberta, where they are similar in age to the ca. 1.986 to 1.960 Ga intrusive suite of the Taltson Magmatic Zone (Bostock et al., 1987; Stern et al., 2003). The granitic gneisses range in grain size from 0.5 to 4 mm and contain 5 to 10% biotite. Some of these exhibit millimetre- to <1 millimetre-scale layers of injected leucosome that impart a gneissic fabric. Domains of paleosome between these layers are equivalent to the granodiorite and quartz diorite

Figure 6 - Migmatitic pelite containing nearly recumbent folds from BAN-9 (Table 1). Note the large garnet porphyroblasts and the leucosome/melanosome relationships and the nearly recumbent folding of the migmatitic gneissosity (left).

Figure 7 - Homogeneous, medium-grained granodiorite to quartz diorite (right) intruded by granite in BAN-8 (Table 1).


described above. In other cases the granitic gneisses are migmatitic, with clear leucosome-melanosome segregations and no indication of whether the protolith was igneous or sedimentary. Based on the relative homogeneity of these intersections, however, a granitoid protolith is likely.

Unfoliated to mylonitic peraluminous granite comprises much of the younger and more prevalent of the two intrusive suites in the north part of the Carswell Structure. Examples range in grain size from 2 to 5 mm and contain 1 to 4% garnet that has generally been altered to biotite, and 1 to 5% discrete biotite grains. Massive granite has grain sizes varying from medium to pegmatitic and contains 1 to 10% biotite and 1% pyrite. Rocks in this unit are typically pink and can be distinguished from nearly massive versions of the peraluminous granite by their lack of garnet, although emplacement may have been contemporaneous.

5. Unit Descriptions: Clearwater Domain The granites exposed in the Clearwater River valley lie at the margin of the Clearwater aeromagnetic high. They are only weakly magnetic, except where there has been mixing between different magma phases (Card, 2002), and hence may not accurately characterize the rocks that underlie the central part of the Clearwater Domain. Although many of the diamond drill holes collared over the Clearwater aeromagnetic high intersect highly altered and effectively unrecognizable rocks, two holes in which relatively fresh rocks are preserved contain two phases of basement granitoids.

K-feldspar-phyric granite is the most common rock type in both cores and is probably correlative with the ca. 1843 Ma K-feldspar-phyric granites exposed in the Clearwater River gorge (Stern et al., 2003). It contains weakly altered K-feldspar phenocrysts up to 10 mm long, 5 to 15% biotite, and traces of magnetite. One noticeable feature is the increase in magnetite content with distance from the unconformity. Just beneath the unconformity rocks are barren; however, magnetic susceptibility readings increase with depth as less altered rocks are encountered and were highest where this unit was in contact with medium-grained to pegmatitic granites. Medium-grained to pegmatitic granites that intrude the K-feldspar-phyric granite contain 5 to 10% combined biotite and muscovite and 1 to 2% hematite.

6. Metamorphism and Structure The basement to the western Athabasca Basin is generally highly metamorphosed, with only the youngest intrusive rocks escaping the peak conditions. Based on the ca. 1985 Ma minimum age of the Careen Lake Group (Card et al., in press) it is possible that these rocks were affected by intense thermotectonic events at ca. 2.6 to 2.5 Ga (e.g. Hanmer, 1997; R. Hartlaub, unpubl. data) and/or ca. 2.3 Ga (R. Hartlaub, unpubl. data). It is certain, however, that the Careen Lake Group and the granodiorites and quartz diorites that intrude it were subjected a ca. 1.94 to 1.90 Ga metamorphic event (Stern et al., 2003). Migmatitic relationships observed in many of the metasedimentary lithologies indicate that the Careen Lake Group was metamorphosed to at least upper amphibolite facies. Abundant blue quartz suggests granulite facies metamorphism (e.g. Niggli and Thompson, 1979). Near the Virgin River Shear Zone, the granodiorites and quartz diorites were metamorphosed to granulite facies between 1.94 and 1.90 Ga (Stern et al., 2003). To the west in BAN-series core, SYL-1 (Table 1), and in examples from Alberta, these rocks are weakly deformed and lack a gneissic texture. Furthermore, prograde cordierite is not observed in the exposed pelites of the eastern Lloyd Domain whereas it is common at Cluff Lake. These factors indicate an eastward increase in both temperature and pressure culminating in granulite facies assemblages near the Snowbird Tectonic Zone similar to the models proposed for the area north of the Athabasca Basin (e.g. Kopf and Williams, 1999). This may reflect southeast-verging reverse displacement along the Snowbird Tectonic Zone (Card, 2002; Mahan et al., 2003). The voluminous peraluminous granite found in the region is likely the product of anatexis during the ca. 1.94 to 1.90 Ga metamorphic episode and this is corroborated by ca. 1.93 to 1.91 Ga U-Pb ages on similar granitoid rocks from the Shea Creek area (Brouand et al., 2003). The extent of Trans-Hudson metamorphism in the region is uncertain, however, ca. 1.84 to 1.83 Ga granitoid rocks in the Clearwater and southeastern Lloyd domains indicate that the effects of the Trans-Hudson Orogeny were far reaching (Stern et al., 2003; Card et al., in press). It is certain that significant movement along major shear zones in the region, including the Snowbird Tectonic Zone, occurred during the Trans-Hudson Orogeny (e.g. Bickford et al., 1994; Card et al., in press).

Pelitic and some psammopelitic members of the Careen Lake Group exhibit a moderately dipping migmatitic gneissosity (S1), whereas psammites, quartzites, and most psammopelites lack leucosome and contain a well-developed, moderately dipping foliation. Rare near-recumbent folding (F2) of these early fabrics is superposed by near-upright folds (F3) with axial planes essentially perpendicular to the early foliation. These late folds are commonly accompanied by an axial planar foliation defined by biotite and chlorite.


In contrast, the intrusive rocks are less well foliated. Members of the granodiorite to quartz diorite suite display a strong, gently to moderately plunging linear fabric, except where injected sheets of granite impart a gneissosity. In most cases the younger peraluminous granites also contain a weak foliation.

It is unclear how the fabrics preserved in the Careen Lake Group relate to those in the intrusive rocks but two scenarios are plausible. In the first, the older gneissic fabrics in the Careen Lake Group were the product of a thermotectonic event predating the intrusive rocks. If the Careen Lake Group is Archean it may have been affected by ca. 2.55 to 2.50 and 2.30 Ga thermotectonic events (e.g. R. Hartlaub, unpubl. data). The younger rocks and the fabrics preserved within would be the products of the ca. 2.0 to 1.9 Ga Taltson-Thelon and the ca. 1.9 to 1.8 Trans-Hudson orogenies. In the second scenario all of the fabrics are the product of the latter two orogenic events. In this model the differences in fabrics between the metasedimentary rocks and the older of the two intrusive suites result from differences in rock strength properties. The youngest peraluminous granites were derived during ca. 1.94 to 1.90 Ga high-grade metamorphic events, and thus were subjected to only the waning stages of deformation.

Centimetre- to metre-scale ductile shear zones were commonly observed and appear to affect all of the rocks investigated during this study. Mylonitic foliations are enhanced by spectacular quartz ribbons. The mylonites postdate folding and lack the sugary texture of annealed mylonites, indicating that they are among the youngest ductile features preserved in the region. The ductile deformation was followed by significant brittle-ductile to brittle deformation, commonly superposed on the older mylonite zones. Millimetre-scale shear zones commonly occur as conjugate sets with one set parallel to the dominant regional foliation and the second nearly vertical, indicating a steeply plunging principal stress direction that would have initiated normal faulting along pre-existing discontinuities. Chlorite lining the shear zones indicates that they formed under greenschist facies conditions.

Evidence from the basement and lower part of the Athabasca Group indicates that there was local relief due to normal faulting at the time of deposition of the basal part of the Athabasca Group (Figure 8). Drill holes HK-7, -1, -2 and -3 (Table 1) comprise a southeast-northwest fence across a northeast-trending fault zone running through the northwest part of Hook Lake (Assessment file 74F15-SW-0013R). The lower Athabasca Group in HK-7 and -1 contains relatively clean quartz arenites with sporadically spaced millimetre- to centimeter-scale pebble beds above the unconformity, but lacks a basal conglomerate. In contrast, the lower Athabasca Group in holes HK-2 and -3 contains a basal conglomerate. HK-2, the easternmost of the two holes, contains coarse, relatively angular quartzite clasts suggesting a proximal source, whereas quartz pebbles and cobbles in HK-3 show more evidence for transport. Deposition of coarse basal clastics northwest of the fault suggests uplift on its southeast side. Furthermore, HK-2

Figure 8 - Cartoon cross-section across a normal fault in the Hook Lake region with the approximate locations of HK-1, -2, and 3 with respect to the observed basal facies of the Athabasca Group indicated.


contains interbedded siltstones and sandstones above the basal conglomerate in contrast with HK-3 which contains only sandstones, indicating that this local basin shallowed to the northwest. Although the dip of the fault cannot be ascertained directly, more intense basement alteration in HK-2 relative to HK-1 (Figure 8) suggests a northwest-dipping fault, along which fluids were transmitted causing more intense alteration in the hanging wall. These data suggest that a half graben developed along a northwest-dipping normal fault from which coarse clastics were derived. The result was likely a linear paleovalley with a local deposystem not unlike those identified elsewhere in the Athabasca Basin (e.g. Harvey and Bethune, 2000). Locally, uranium mineralization (e.g. Deilmann; Harvey and Bethune, 2000) is related to such paleotopographic features.

Evidence for brittle faulting in HK-series cores and in the northern Carswell Structure is ubiquitous, although some of the faulting observed in the Carswell Structure is clearly the product of thrusting during uplift of the basement core of the astrobleme (Pagel et al., 1985). Breccia zones are commonly superimposed on most of the observed lithologies. Where they intersect pelitic rocks, graphite, probably responsible for basement geophysical conductors, is commonly concentrated. Quartz-carbonate veins are also common, especially in the rare mafic lithologies (Figure 4). They occur as conjugate sets that indicate a nearly vertical principal stress, compatible with normal faulting. The main problem with interpreting the age of these structures is the lack of reliable chronostratigraphic constraints and the inability to determine the number of offset episodes. The timing of post-Athabasca faulting will be further discussed below in conjunction with our preliminary observations on radar topography images.

7. Basement Summary and Discussion With impending conclusion of the EXTECH IV project, a short summary of the findings from the Western Athabasca Basin Basement sub-project is warranted. A more detailed discussion will be presented in the EXTECH IV results volume (Card et al., in press). Mapping of the basement rocks in conjunction with U-Pb SHRIMP age determinations provide the framework for this discussion. The oldest rocks identified in this study are ca. 2.5 Ga granitic gneisses from the Clearwater Domain. It is difficult to discern their relationship with the other rocks of the Lloyd Domain because comparable granitoid gneisses have not been positively recognized elsewhere in the domain, although Scott (1985) does refer to rare granitoid gneisses. It seems unlikely that they are restricted to the relatively narrow Clearwater Domain unless they were carried as rafts from depth during emplacement of the younger Clearwater granites.

Hence the relationship between the Clearwater gneisses and the undated Careen Lake Group is uncertain, although Scott (1985) considered the granitoid gneisses older. A possible interpretation is that the Careen Lake Group represents a southern extension of the Murmac Bay Group, a supracrustal package believed to be Archean and widespread in the Uranium City area (Hartlaub et al., 2002), that extends east to the Tantato Domain (Card, 2002) and is preserved as dismembered relicts in a sea of orthogneiss west of the Black Bay Fault (Ashton et al., 2001). Alternatively, the Careen Lake Group may be correlative with supracrustal rocks of the 2.17 to 2.09 Ga Rutledge River rift basin of the Taltson Magmatic Zone (Bostock and van Breemen, 1994) or the ca. 2.075 Ga lower Wollaston Group (Hamilton and Delaney, 2000). Equivalence with the post-1.90 Ga upper Wollaston Group (Tran et al., 2003) is precluded by the ca. 1.985 Ga minimum age constraint imposed by the intrusive quartz diorite at Careen Lake (Stern et al., 2003).

Circa 1.985 to 1.968 Ga granodiorites and quartz diorites (Stern et al., 2003) are most common in the southeastern Lloyd Domain but also occur in the vicinity of the Carswell Structure and in northeastern Alberta (Stern et al., 2003). Their ages overlap those of the ca. 1.986 to 1.960 Ga calc-alkaline intrusive suite of the Taltson Magmatic Zone (Bostock et al., 1987; McDonough and McNicoll, 1997; McDonough et al., 2000) and indicate that Taltson-age rocks are far more extensive than originally surmised. These rocks had generally been considered Archean by most previous workers (e.g. Scott, 1985). Many of the younger peraluminous granites appear to derive from anatexis during 1.94 to 1.90 Ga metamorphism associated with the Taltson-Thelon orogen and are not related to the Tran-Hudson Orogen to the east (e.g. Brouand et al., 2003).

These results are consistent with poorly constrained ca. 2.00 to 1.95 Ga ages for gneissic samples from the eastern Lloyd Domain (Bickford et al., 1994). Of particular interest in that work was a sample of tonalitic gneiss collected from a xenolith within the Clearwater anorthosite complex. Its U-Pb age of 2006 ±34 Ma is within the error of the 1.986 to 1.960 Ga age range for the calc-alkaline suite of the Taltson Magmatic Zone (Bostock et al., 1987) and the quartz diorite suite at Careen Lake (Stern et al., 2003) and hence it is likely that this rock is part of the same suite. The most recent attempts to date the Clearwater anorthosite complex yielded a 1908 ±2 Ma U-Pb zircon age (Heaman et al., 1999) and a 1917 ±7 Ma Ar-Ar hornblende cooling age (Halls and Hanes, 1999), both of which were interpreted to be metamorphic ages. The ca. 2.00 Ga age for the inclusion and the ca. 1.92 Ga metamorphic age constrains the age of the Clearwater anorthosite complex to within 80 Ma, although it is unclear if it was associated with the ca. 1.986 to 1.960 Ga calc-alkaline or ca. 1.95 to 1.92 Ga syn-metamorphic plutons (Bostock et al., 1987; McDonough et al., 2000), if either.


The presence of Taltson Magmatic Zone intrusive rocks throughout the region allows for a better understanding of the southern Rae Province. Before the onset of this project it was suggested by many workers that the rocks of the Tantato Domain were continuous with those of the Lloyd Domain beneath the Athabasca Basin and represented a tectonic lozenge uplifted along the Snowbird Tectonic Zone (Hoffman, 1990; Hanmer et al., 1994), mainly on the basis of the apparent continuity of aeromagnetic lineaments This appears not to be the case. Apart from the geophysical arguments (Hanmer et al., 1994), geochronological work indicates that both the supracrustal package and intrusive packages of granodioritic to tonalitic and mafic to ultramafic affinities of the Tantato Domain are Archean in age and were subjected to ca. 2.6 and 1.9 Ga thermotectonic overprints (Hanmer, 1997; Baldwin et al., 2003).

Much of the Clearwater Domain is apparently underlain by distinctive megacrystic granite. SHRIMP U-Pb geochronology indicates that these granites are ca. 1.843 Ga in age (Stern et al., 2003). The texturally similar ca. 1.82 Ga Junction granite (Bickford et al., 1994) lies within the Snowbird Tectonic Zone as does a ca. 1.83 Ga aplitic granite from Careen Lake (Stern et al., 2003), suggesting that the granites may have used pre-existing structural discontinuities as conduits. To date, no such structure has been associated with the Clearwater granites, although a north-trending magnetic fabric in the southern Clearwater Domain is a structural overprint of the magnetic signature of the Lloyd Domain and may indicate a wide fault zone along which the granites were emplaced.

The Virgin Schist Group is a sliver of lower to middle amphibolite facies supracrustal rocks dominated by metasedimentary lithologies and restricted to the Virgin River Shear Zone. It is surrounded by dominantly upper amphibolite to granulite facies orthogneisses suggesting that its preservation is a function of west side-up displacement along the Snowbird Tectonic Zone (Card, 2002). The provenance and age of the Virgin Schist Group is unknown, although the rocks are similar to supracrustal rocks in both the Virgin River and Lloyd domains (Wallis, 1970; Card et al., in press).

8. Regional Lineament Analysis Remote image analysis is a useful means for assessing the post-depositional structural history of the Athabasca Basin. Two types of images of the Athabasca region (1:500 000 scale) were used in this preliminary lineament analysis: 1) Shuttle Radar Topography Mission (SRTM) and 2) Landsat 743. The SRTM data has proven the most useful. The technical details of SRTM images can be found on NASA’s website (http://www.jpl.nasa.gov/srtm/ factsheet_pub.html), but in brief the SRTM data relating to this project were collected during an 11-day shuttle mission in 1999. Two simultaneous radar images were obtained by two independent radar antennas and were combined to produce a 3-D topographic image with 90 m resolution. The digital elevation models (DEM) used in this study are 3-D topographic maps created from these images.

A summary on Landsat data collection can be found on NASA’s website at http://landsat.gsfc.nasa.gov/project/ satellite.html. The Landsat 7 satellite was launched in 1999 and orbits the earth at an elevation of 705 km. It collects three types of earth scene radiation data: visible and near infrared, which includes bands 1 to 4 and 8; short-wavelength infrared, which includes bands 5 and 7; and thermal long-wavelength infrared, which contains band 6. The data from different bands can be combined to enhance image quality. For the purpose of this study bands 7, 4, and 3 have been combined (i.e., a Landsat 743 image) to create an image with 30 m resolution. Although the major structural features of the western Athabasca Basin can be identified, Landsat 743 images are not as useful as the SRTM images because their spectral signatures are influenced by moisture and vegetation, which obscure the bedrock features.

Post-Athabasca Group reactivation of major basement structural features underlying the western Athabasca Basin, which are recognizable on the Geological Survey of Canada (1987) aeromagnetic maps (Figure 2), played a role in controlling regional Quaternary features. Basement structures controlled the pre-glacial bedrock topography, which in turn controlled ice-flow directions, sub-glacial melt water drainage, and development of associated geomorphic features. Major structural features that are apparent include the Snowbird Tectonic Zone, Grease River Shear Zone, Beatty River Fault, Cable Bay Shear Zone, and the Tabbernor Fault system (Figure 9). Most of the lineaments observed on the images can be attributed to the Snowbird Tectonic Zone and the Tabbernor Fault system.

The Snowbird Tectonic Zone is one of the longest linear features in the Canadian Shield, stretching northeastward from the Rocky Mountains to the Baker Lake area of Nunavut (Hoffman, 1988). In Saskatchewan, it is exposed along the Virgin River and Black Lake shear zones, respectively south and north of the Athabasca Basin. On the aeromagnetic map these shear zones appear to join beneath the Athabasca Basin on aeromagnetic maps (Figure 2; Pilkington, 1989). The surface projection of the Snowbird Tectonic Zone to the southeast of its magnetic trace is consistent with it being a northwest-dipping structure. Recent work on the 5 to 8 km wide, moderately to steeply northwest-dipping Black Lake Shear Zone (Legs Lake Shear Zone of Mahan et al., 2003) suggested that it accommodated west-side-up dextral-oblique displacement (Mahan et al., 2003). Similarly, Card et al. (in press) suggest that the Virgin River Shear Zone evolved from a southeast-verging thrust into a dextral strike-slip shear


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zone with at least two displacement episodes, the last of which postdates 1.82 Ga (Bickford et al., 1994; Card et al., in press). Lineaments parallel or sub-parallel to the Snowbird Tectonic Zone are common across the western Athabasca region and likely formed within the same stress regime that initiated regional dextral displacement (Figure 9, 10, 11, and 12). The northeast- to east-northeast-trending Grease River Shear Zone is also a dextral shear zone that merges with the Black Lake Shear Zone at the apex of the Tantato Domain (Hanmer, 1997). Both the displacement sense of the Grease River Shear Zone and its orientation with respect to the Black Lake Shear Zone are compatible with it being a large asymmetric extensional shear band or a Riedel (R) shear (Hatcher, 1995) associated with the Snowbird Tectonic Zone. Lineaments parallel to the Grease River Shear Zone, such as the Beatty River Fault, are also common features in the Athabasca Basin (Figure 9, 10, 11, and 12) indicating that R-shears are a common feature of the Snowbird Tectonic Zone. The surface expression of the Grease River Shear Zone, seen clearly on the SRTM DEM images, controls the outcrop pattern of the Locker Lake Formation of the Athabasca Group (Ramaekers et al., 2001) indicating that displacement postdated deposition of the Athabasca Group. Furthermore, the aeromagnetic signature of a Mackenzie dyke shows an apparent sinistral displacement along the fault in (Figure 2), indicating post-1.27 Ga reactivation (LeCheminant and Heaman, 1989).

West-northwest- to northwest-trending lineaments are also common, in particular to the south of the Tantato Domain (Figure 10), and are parallel to lineaments within the Tantato Domain. They have an orientation compatible with being anti-Riedel shears (R’) (Hatcher, 1995) associated with the Snowbird Tectonic Zone and they control Quaternary features such as drumlins and eskers.

The north-trending Tabbernor Fault spans the exposed Canadian Shield in northeastern Saskatchewan, and extends southward beneath Phanerozoic cover. Along the exposed Tabbernor Fault, the latest episode of ductile displacement was sinistral and bracketed between 1.848 to 1.737 Ga (Elliot, 1995). Sinistral displacement of aeromagnetic lineaments is common along north- to north-northwest-trending structures in northeastern Saskatchewan and farther west in the Lloyd Domain (Geological Survey of Canada, 1987). Three main directions of lineaments attributed to the Tabbernor Fault system can be recognized on the SRTM images of the Athabasca Basin: 1) north-trending lineaments, parallel to the master fault; 2) north-northwest trending R-shears; and 3) west- to west-northwest- trending, which may represent R’-shears (Figure 10, 11 and 12).

It should be emphasized that the present-day lineament patterns observed did not necessarily result from fault displacement under the same regional stress regimes that initiated the major regional fault systems. In many cases, it can be demonstrated that structures have been subjected to multiple stress regimes during their displacement history. These fault systems do, however, represent regional discontinuity networks, which were repeatedly reactivated and played a significant role in the regional metallogenic framework. A good example is the Snowbird Tectonic Zone, which was apparently initiated as a thrust fault before changing into a dextral shear system. Subsequent to these displacement events, quartz-filled tension gashes indicate a sinistral component of shear. Drill core analysis near the margin of the Athabasca Basin along the Dufferin Lake Fault, which is superimposed on the Virgin River Shear Zone, indicates episodes of both normal and reverse displacement (Yeo et al., 2002), the youngest of which post-dates emplacement of the Mackenzie intrusions (Card et al., in press). Although there are few time constraints on the age of fault reactivation, indirect evidence from the SRTM DEM images can help to establish a history of fault displacements: 1) activity along both of the aforementioned fault sets postdates deposition of the Athabasca Group, including the youngest formations preserved in the annulus surrounding the Carswell Structure (Figure 12); 2) faults of the Snowbird Tectonic Zone system displace the ca. 1.27 Ga Mackenzie dykes; 3) a lineament parallel to the Grease River Shear Zone and north-trending faults of the Tabbernor system cut across the boundary of the Carswell Structure indicating post-Ordovician displacement (Pagel et al., 1985); and 4) uranium deposition and remobilization events at ca. 1540, 1247, 950 (Alexandre and Kyser, 2003) and 300 Ma (Thomas et al., 2000) are evidence for fluid movement associated with fault displacement.

9. Conclusions 1) Rocks of the Careen Lake Group are ubiquitous across the southern basement to the western Athabasca Basin,

justifying the amalgamation of the former Firebag and Western Granulite domains into the Lloyd Domain (Card, 2002). The age of these supracrustal rocks is not well established but they predate ca. 1.985 Ga magmatism associated with the Taltson-Thelon orogen.

2) A suite of 1.985 to 1.968 Ga intrusive rocks previously identified in the Careen Lake area south of the Athabasca Basin and in Alberta significantly broadens the extent of the Taltson Magmatic Zone eastward.

3) Abundant peraluminous granites in the Carswell Structure and near Hook Lake are likely the equivalent of ca. 1.93 to 1.91 Ga granites in the Shea Creek area and were likely generated and injected during a high-grade metamorphic event associated with the Taltson-Thelon orogen. This metamorphic event is the only well-constrained one in the region, although it is likely that there were both earlier and later thermotectonic events.


Figure 10 - SRTM DEM image of the area just to the south of the Tantato Domain. Structural features indicated on the image: 1, surface expression of the Grease River Shear Zone; 2, north-trending lineaments of the Tabbernor Fault system; 3, northeast-trending valley parallel to the Snowbird Tectonic Zone; 4, west-northwest-trending lineaments parallel to the probable R’-shears related to the Snowbird Tectonic Zone; 5, lineament parallel to the Grease River Shear Zone or R-shear related to the Snowbird Tectonic Zone; and 6, surface expression of the Snowbird Tectonic Zone.

4) Northeast-trending normal faults led to localized linear basins that influenced the deposition of the basal Athabasca Group in the Hook Lake area.

5) Preliminary lineament analysis of the western Athabasca Basin using Landsat and SRTM DEM images indicates that they are useful tools in interpreting the regional structural history. The Snowbird Tectonic Zone and the Tabbernor Fault system appear to be responsible for many of the linear features preserved on the Athabasca Basin.


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Figure 12 - SRTM DEM image of the Carswell Structure area. Structural features indicated on the image: 1, north-trending lineaments parallel to the Tabbernor Fault system; 2, lineament parallel to the Grease River Shear Zone or R-shear related to the Snowbird Tectonic Zone; and 3, northeast-trending valley parallel to the Snowbird Tectonic Zone.

10. Future Directions 1) Basement compilation map including data accumulated in this study, older compilation work (e.g. Gilboy,

1982), and assessment file data. 2) Compilation of structures using SRTM-DEM and Landsat images. 3) Structural and stratigraphic evidence for syn-Athabasca fault displacement. 4) Use of isotopic systems such as 40Ar-39Ar to constrain the displacement ages for syn- to post-Athabasca fault

displacement.


11. Acknowledgments Meghan Hendren provided cheerful and able assistance, both in the field and in the office. Barry O’Brien and Voyage Air in Buffalo Narrows are thanked for their reliable service and their patience. The staff at the Cluff Lake Mine welcomed us this summer and their enthusiastic response to our presence is appreciated. Erwin Koning of COGEMA Resources Inc. and Dan Jiricka of Cameco Corp. are thanked for helping with field logistics. Thanks to Dave Thomas of Cameco Corp. for encouraging the analysis of remote imagery.

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