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. Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 2 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% Figure 1 - Map of geological domains in Saskatchewan and northeastern Alberta, biotite and 1% garnet including the approximate location of the largely unexposed Clearwater Domain and the major structural features. Area 1, Hook Lake; Area 2, Northern Carswell porphyroblasts. Intensive Structure; LD, Lower Deck; and UP, Upper Deck. alteration generally resulted in Saskatchewan Geological Survey 2 Summary of Investigations 2003, Volume 2 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). 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 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. Saskatchewan Geological Survey 3 Summary of Investigations 2003, Volume 2 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.