Geology of the Camsell Portage Area, Southern Zemlak Domain, Rae Province (Uranium City Project)

K.E. Ashton and R.C. Hunter 1

Ashton, K.E. and Hunter, R.C. (2004): Geology of the Camsell Portage area, southern Zemlak Domain, Rae Province (Uranium City Project); in Summary of Investigations 2004, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004-4.2, CD-ROM, Paper A-8, 12p.

Abstract The Camsell Portage area includes a variety of foliated to gneissic granitoid rocks of inferred Archean age, psammopelitic to pelitic gneisses and migmatites, and widespread leucogranites, all of which were subjected to upper amphibolite facies metamorphism at about 1.9 Ga and multiple deformational events prior to intense and widespread mylonitization. Following exhumation, this basement complex was unconformably overlain by the Thluicho Lake Group, a fining-upward conglomerate-arkose-argillite succession that was metamorphosed to greenschist facies and doubly folded prior to uplift, erosion, and deposition of the Martin Group redbeds. Deposition of the Martin Group was accompanied by widespread diabase dyke emplacement at about 1.82 Ga along dominantly east-west normal faults, and both were subjected to north-trending folds attributed to the Trans- Hudson Orogen.

A number of Cu-Pb-Ag±U showings are associated with the basal unconformity of the Thluicho Lake Group and the east-trending diabase dykes. The basin into which the Thluicho Lake Group was deposited appears to have originally been much larger, enhancing the potential for economic mineralization.

Keywords: Zemlak Domain, Rae Province, Rae-Hearne Craton, Churchill Province, Uranium City, Thluicho Lake Group, Thelon-Taltson Orogen.

1. Introduction The 2004 mapping component of the Uranium City Project (Hartlaub and Ashton, 1998; Hartlaub, 1999; Ashton et al., 2000, 2001; Ashton and Hunter, 2003) extended coverage 30 km westward from the Wellington Lake Power Station, located approximately 25 km west of Uranium City, to McKenzie Point along the northern shore of Lake Athabasca (Figures 1 and 2). This 1:20 000 scale mapping ties on to previous coverage, extending the east-west

110° o 108 Northwest Territories 104° 60° 60° NOLAN lt Z T ENA S Bay Fau DODGE ck TRAIN ZEMLAK Bla OBS Zone SLF ar Z e h ta MUDJATIK ON r se River Shear Zone S e Grea NORTH LTS Uranium LAKE b ake l A TO City BEAVERLODGE L T TA (HEARNE A k c TAN la PROVINCE) B Lake Athabasca Fond-du-Lac Black ATHABASCA BASIN Stony Rapids Lake 0 km 50 59° 59° 110° 104°

Figure 1 - Location map showing lithotectonic domains in the Rae Province of northwestern Saskatchewan; 1: 20 000 scale mapping in 2004 outlined by heavy black line; areas previously mapped as part of Uranium City and Rae Northeast projects denoted by thin grey lines; SLF, St. Louis Fault; OBSZ, Oldman-Bulyea Shear Zone; and STZ, Snowbird Tectonic Zone.

1 Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2.

Saskatchewan Geological Survey 1 Summary of Investigations 2004, Volume 2

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Saskatchewan Geological Survey 2 Summary of Investigations 2004, Volume 2 transect of the Beaverlodge (Hartlaub and Ashton, 1998; Hartlaub, 1999; Ashton et al., 2000) and Zemlak (Ashton et al., 2001; Ashton and Hunter, 2003) lithotectonic domains north of Lake Athabasca. Air strips servicing Camsell Portage and the Wellington Lake Power Station provide access for wheeled aircraft and the area can also be reached by boat from Uranium City. The eastern part of the area is characterized by mature bush and rugged topography with a 225 m range in relief. The west is characterized by more subdued topography and has experienced a recent forest fire. Most of the area has received little attention since the 1950s (Hale, 1954a, 1955).The easternmost part was mapped in 1972 to provide a regional context for numerous Cu-Ag-Pb showings (Scott, 1978), whereas the far west was mapped in 1965 (Koster, 1967). The southwestern extent is near-contiguous with 1978 to 1979 mapping in the Maurice Bay area (Harper, 1996).

2. Regional Geology The oldest rocks include: 1) variably magnetic orthogneisses, 2) pelitic gneissic to migmatitic rocks and derived diatexites, 3) locally garnetiferous leucogranites, and 4) rocks that are too altered and mylonitized to classify. The quartzite-amphibolite association characterizing the Murmac Bay Group (Hartlaub, 2004) was not recognized, so it is unclear whether the dominantly pelitic supracrustal rocks mapped are correlative or part of a distinct succession. Following upper amphibolite facies metamorphism at about 1.9 Ga (Ashton et al., 2004) and multiple folding, these rocks underwent widespread and intense mylonitization. The extent and cause of this deformation are unclear. To the east, it dies out along a number of splays (Ashton and Hunter, 2003), whereas it appears truncated to the west by brittle faulting. Its northern extent is open, although a gradational boundary was established in the northwest. There are at least subtle lithological differences between the rocks hosting the mylonite zone and those exposed farther east, so more work will be necessary to determine its significance.

Following mylonitization, these basement rocks were exhumed and apparently exposed to chemical weathering prior to deposition of the Thluicho Lake Group (Scott, 1978; Hunter et al., 2003), a fining-upward fluvial succession comprising basal polymictic conglomerate, abundant arkose, and argillaceous rocks. The Ellis Bay Formation, which Scott (1978) interpreted as a distinct lithological unit unconformably overlying the Thluicho Lake Group, is re-interpreted as a tectonic breccia derived from mainly Thluicho Lake Group argillaceous strata via brittle faulting, possibly accompanied by sedimentary diatreme activity (Hunter et al., this volume).

Diagenesis of the Thluicho Lake Group subjected the basement rocks to further alteration, so that subsequent greenschist facies metamorphism converted the basement gneisses directly below the Thluicho Lake Group unconformity into chlorite-sericite schists and phyllonites. Deformation associated with this younger metamorphic event effectively transposed shear fabrics and kinematic indicators related to the older mylonitic event.

Renewed uplift and exposure following this greenschist facies metamorphism and associated deformation preceded deposition of the Martin Group (Tremblay, 1972, Mazimhaka and Hendry, 1984, 1985). Circa 1.82 Ga east- trending diabase dykes intruding the folded Thluicho Lake Group constrain both the succession and the folding to the 1.90 to 1.82 Ga age range. These dykes are also considered correlative with the Martin Group basalts, indicating that deposition of the succession was ongoing at 1.82 Ga. The last phase of regional folding, which deforms both the unmetamorphosed Martin Group and the diabase dykes, is attributed to the Trans-Hudson orogeny. Most of the brittle faulting is thought to have taken place at this time, including the major east-trending set, which defines much of the northern shoreline of Lake Athabasca. This late faulting is partly responsible for preservation of the Thluicho Lake Group, the Martin Group, and the flat-lying Athabasca Group, which was deposited following the cessation of regional folding. The abundance of bedrock exposure has facilitated widespread development of erosional glacial features. Striae record four ice-flow directions listed from oldest to youngest: 229°, 209°, 256°, and 224°.

3. Unit Descriptions a) Granitoids of Probable Archean Age A variety of foliated granitoids and locally migmatitic orthogneisses form much of the basement to the Thluicho Lake Group. Most outcrops are characterized by variable amounts of injected leucogranite of probable Paleoproterozoic origin and minor amphibolite layers and inclusions, but the host granitoids are tentatively considered Archean, pending geochronological data.

The granitic orthogneiss-migmatite is best exposed and preserved in the northwest beyond the mylonite zone (Figures 2 and 3). Most outcrops are magnetic and include: 1) about 20% granitic gneiss comprised of a grey

Saskatchewan Geological Survey 3 Summary of Investigations 2004, Volume 2 medium-grained paleosome containing 10 to 20% biotite ± hornblende and a centimetre-scale pink medium-grained leucosome; 2) 70% injected pink, medium-grained leucogranite; and 3) 10% semi- continuous amphibolite layers and inclusions (Figure 3). Zones of massive to weakly foliated, medium- grained, hornblende-bearing granitoid, representing either hybrid or melt phases, are more locally developed. In the north, the unit contains a higher proportion of mafic material and the felsic components weather white rather than pink, suggesting a more leucotonalitic composition. Unlike their pink counterparts, such rocks tend to be non-magnetic. The granitic orthogneiss-migmatite extends southward into the main mylonite zone, where it forms distinctive layered pink and grey mylonitic to ultramylonitic gneiss (Figure 4). Minor pink centimetre-scale oval Figure 4 - Mylonitized granitic orthogneiss-migmatite from feldspar porphyroclasts, presumably derived from about 2 km northwest of Slate Island (UTM 588849E, dismembered pegmatite dykes, are commonly 6610489N, NAD83); note centimetre-scale feldspar preserved in the mylonitized granitic orthogneiss- porphyroclasts. migmatite, as is hornblende blastesis. Allanite is a common accessory.

Gneissic granodiorite is common in the southeast. It comprises a medium-grained to rarely megacrystic, grey paleosome and up to 30% pink, centimetre-scale, partial melt leucosome, and typically contains 15 to 25% combined biotite and hornblende with accessory allanite (Figure 5). Most outcrops are injected by about 20% metre-scale sheets of pink medium- to coarse- grained granite and contain about 10% amphibolite inclusions presumably derived from dykes. The gneissic granodiorite is moderately to non-magnetic and mylonitized (Figure 6).

Non-magnetic tonalitic to gabbroic rocks were recognized locally near the margins of large gneissic granodiorite bodies. They weather dark green but are black and white on fresh surfaces, and range from fine Figure 5 - Strain gradient in weakly mylonitized gneissic granodiorite from 1.5 km northeast of Slate Island (UTM to medium grained where layered, to medium to coarse 591730E, 6609099N, NAD83). grained where homogeneous. Like the gneissic granodiorite, they are generally injected by 20 to 30%

Figure 6 - Ultramylonitized gneissic granodiorite from the same outcrop as Figure 5 located 1.5 km northeast of Slate Island (UTM 591730E, 6609099N, NAD83); note beaded Figure 3 - F3/5 ‘Z’ fold in granitic orthogneiss from about feldspar porphyroclasts, hornblende porphyroblasts derived 2 km north of Halfway Point (UTM 586167E, 6610365N, via blastesis, and large δ-porphyroclast inferring dextral NAD83). sense of shear.

Saskatchewan Geological Survey 4 Summary of Investigations 2004, Volume 2 pink leucogranite. Most are tentatively interpreted as border phases of large granodioritic plutons. b) Supracrustal Rocks Together with the foliated to gneissic granitoids, non- magnetic psammopelitic to pelitic gneisses and migmatites dominate the basement to the Thluicho Lake Group. Recognition and subdivision of the metasedimentary rocks is hampered by the effects of mylonitization, but two main units have been distinguished: one is characterized by rare metre-scale beds of quartzite, whereas the other contains abundant ubiquitous garnet. The psammopelitic to pelitic gneiss-migmatite with rare interbedded quartzite is grey brown, fine to medium grained and contains 20 to 30% biotite with rare garnet and sillimanite. Sheets of white to rarely pink, medium-grained, anatectic Figure 7 - Pelitic diatexite from 1 km north-northeast of leucogranite comprise as much as 90% of some Slate Island (UTM 590829E, 6608937N, NAD83). outcrops. Quartzite is white and at one locality contains minor diopside. A diopsidite inclusion at this site suggests that dolostone was originally present. At amphibolite facies, dolomite reacts with quartz to form diopside, and will continue to do so until one of the reactants is consumed.

The psammopelitic to pelitic rocks grade into pelitic migmatite and diatexite in the northwest. These are generally more homogeneous, seriate-textured rocks characterized by medium- to coarse-grained white feldspar in a grey-brown matrix containing about 20% biotite and local garnet (Figure 7). They are remarkably similar to parts of Hale’s (1954b) ‘White Lake Granite’, which have also been reinterpreted as pelitic diatexite (Ashton et al., 2001; Ashton and Hunter, 2003). Mylonitization results in a variable abundance of millimetre-scale feldspar porphyroclasts in a schistose biotite-rich matrix (Figure 8). Figure 8 - Strain gradient in pelitic diatexite showing Garnetiferous pelitic migmatite to diatexite is derivation of very fine-grained, biotite-rich grey restricted to the southeast and represents a westward ultramylonite from pelitic diatexite; from 1.5 km northeast extension of rocks previously mapped (Ashton et al., of Slate Island (UTM 591730E, 6609099N, NAD83). 2001). The best-preserved rocks are white and grey, medium-grained gneisses with a large component of leucosomal melt. Garnet makes up as much as 20% of these rocks and individual grains approach 1 cm or more in diameter (Figure 9), although nearly all have been pseudomorphed by biotite and/or chlorite. These pseudomorphs tend to be flattened and aligned parallel to the axial planes of the last phase of regional folding (Figure 10), and in ultramylonitic schists, they are all that is left of the original rock to indicate a sedimentary origin. In the Low Lake area (Figure 2), a unit of mixed gneisses includes: 1) grey, medium-grained, variably layered biotite gneiss with 10 to 30% biotite and centimetre-scale white leucosome; 2) rare intrusive pink-buff megacrystic granite containing 5 to 15% biotite and amphibolite schlieren; 3) white, locally pegmatitic leucogranite; and 4) pink, medium-grained Figure 9 - Pseudomorphed garnet porphyroblasts in to pegmatitic leucogranite that appears locally mylonitized garnetiferous pelitic migmatite from continuous with the leucosome in the gneisses. The northwestern Island Bay (UTM 606796E, 6601759N, biotite gneiss appears unusually resistant and hard NAD83); note slight flattening perpendicular to layering.

Saskatchewan Geological Survey 5 Summary of Investigations 2004, Volume 2 relative to rocks of similar composition elsewhere and several of the rock types contain blue quartz. Given the map pattern and compositions, the mixed gneisses are probably correlative with the psammopelitic to pelitic gneiss-migmatite unit, but they have apparently been modified. Perhaps they were recrystallized following the amphibolite-facies mylonitic event, but escaped the subsequent greenschist overprint. Minor calcareous quartzite and gneissic to migmatitic pelite outcropping southeast of the main mylonite zone was previously assigned to the Murmac Bay Group (Ashton et al., 2001). The white diopside- and biotite-bearing quartzite is too thinly layered with the grey-brown, fine- to medium-grained pelitic gneisses to distinguish at this scale of mapping, but together they are strikingly similar to other thin Figure 10 - Strongly foliated pseudomorphs derived from remnants of the Murmac Bay Group, including those garnet porphyroblasts oriented in the axial plane of F6 folds, along strike to the northeast where amphibolite is from northwestern Island Bay (UTM 607075E, 6601278N, interlayered with the paragneisses (Ashton and Hunter, NAD83). 2003). Amphibolite occurs as discontinuous layers, schlieren, and inclusions in a variety of rock types, but many of these could represent variably dismembered dykes. The only body mappable at this scale was recognized within mylonites derived from the garnetiferous pelitic migmatite and derived diatexite (Figure 2). It is at least 50 m thick and intruded by abundant pink leucogranite sheets. The fine-grained amphibolite weathers buff and contains 40 to 50% hornblende variably altered to tremolite-actinolite and chlorite.

The age and origin of the supracrustal rocks within the main mylonite zone is unclear. The presence of thin, locally calcareous quartzite within the dominant pelite is suggestive of the Murmac Bay Group, but the areal extent of the mylonitized units is far greater than that of typical Murmac Bay Group remnants west of the Black Bay Fault, and the characteristic quartzite-basalt association was not recognized (Ashton et al., 2001; Ashton and Hunter, 2003). Areally extensive units of similar pelitic migmatite and diatexite are exposed about 75 km to the east in the eastern Beaverlodge Domain (formerly Nevins Lake Block; Ashton and Card, 1998; Hartlaub and Ashton, 1998). The Murmac Bay Group rests unconformably on extensive Archean basement (e.g., Ashton et al., 2001; Ashton and Hunter, 2003) and is now thought to be of Paleoproterozoic age based on the recent discovery of ca. 2.3 Ga detrital zircons in its basal conglomerate (Ashton and Hunter, 2003). Thus, it is possible that the widespread psammopelitic to pelitic rocks of the study area are part of an older Archean succession that forms part of the basement to the Murmac Bay Group. c) Granitoid Rocks of Probable Paleoproterozoic Age A mylonitized, pink, coarse-grained granite situated within psammopelitic to pelitic gneisses near the eastern margin of the mylonite zone was previously described and interpreted (Ashton and Hunter, 2003) as part of a suite of ca. 2.3 Ga granites best known in the Beaverlodge Domain (Persons, 1983; R.P. Hartlaub, pers. comm., 2000). Rocks of this suite were not recognized farther west.

Much of the western part of the area mapped is underlain by protomylonitic, homogeneous to gneissic leucogranite that has undergone intense brittle-ductile shearing and bleaching. Much of the layering in these rocks is attributed to shearing, although their non-magnetic character, minor remnants of pelitic gneiss, and near-ubiquitous garnet all suggest derivation from dominantly sedimentary precursors. Two varieties have been distinguished, although contacts are gradational and their distinction may simply be due to the proximity of pelitic remnants. The garnetiferous anatectic leucogranite variety grades from white to less commonly pink, is medium to coarse grained, and contains up to 10% biotite and chlorite, much of which is derived from the alteration of garnet. It commonly exhibits a pink, centimetre-scale melt leucosome and sheets of injected pink leucogranite are common. The generally non-garnetiferous anatectic granite is more commonly pink grading to white due to bleaching, and locally contains blue quartz, along with up to 10% biotite and chlorite. It is also found as small bodies in the mylonite zone to the east, where it appears less deformed than the surrounding mylonites, possibly inferring syn- to post-tectonic emplacement. It is similar in appearance to pink leucogranites previously mapped east of the mylonite zone (Ashton et al., 2000; Ashton et al., 2001; Ashton and Hunter, 2003). These rocks are tentatively attributed to widespread crustal melting resulting from a high-grade metamorphic event at about 1.9 Ga (Ashton and Hunter, 2003).

Saskatchewan Geological Survey 6 Summary of Investigations 2004, Volume 2 d) Mylonitic Rocks An attempt has been made to classify the mylonitized rocks according to their lithological precursors, but in some cases there was insufficient preservation. This is particularly true in the east where the mylonites appear to have been overprinted by intense brittle-ductile structures indicating formation or reactivation at relatively high crustal levels under greenschist facies metamorphic conditions. They vary from fine-grained, sugary textured rocks exhibiting significant grain-size reduction, feldspar beading, ribbon quartz, and abundant millimetre-scale, chlorite- coated fractures to more micaceous, grey-green phyllonites. Mylonites containing remnants of white granitoid and biotite-rich rocks were previously thought to be derived from white anatectic and massive granites, pelitic diatexites, and minor associated pelitic to psammopelitic paragneisses based on the local presence of pseudomorphed garnet porphyroblasts (Ashton and Hunter, 2003). Since another look at these rocks only served to strengthen that suspicion, these rocks have been grouped with the psammopelitic to pelitic gneiss-migmatite unit, rather than separated as a distinct variety of mylonite. Mylonites derived from pink leucocratic granitoids could represent a variety of precursors. Both major occurrences of these rocks, in the Zemlak and Waterloo lakes areas, are directly along strike from pink leucogranites to the east, but various other pink leucocratic granitoids are known from the area (Ashton and Hunter, 2003), and given the intensely deformed nature of these rocks, some of the recognizable pink material may simply represent small granitic sheets in outcrops dominated by older rock types. The mylonitic rocks of unknown origin include chlorite-sericite schists and phyllonites that have been too compositionally and texturally modified to classify. They probably include both altered granitoid and pelitic rocks. e) Thluicho Lake Group The Thluicho Lake Group rests with local angular unconformity on the mylonitized basement rocks described above. It constitutes a single, fining-upward conglomerate-arkose-argillite succession that was subjected to greenschist facies metamorphism. Full descriptions, along with stratigraphic and structural interpretations are provided in a companion paper (Hunter et al., this volume). f) Martin Group The Martin Group is exposed at several localities along the northern shore of Lake Athabasca, apparently in fault- controlled basins. In the study area, it is a redbed siliciclastic succession comprising dark red- to pink-weathering, coarse paraconglomerate and stratigraphically overlying arkose and siltstone, collectively termed the Charlot Point Formation (Mazimhaka and Hendry, 1985). The clast-supported conglomerate is poorly sorted with granule- to boulder-size clasts in a very fine- to silt-sized matrix (Figure 11). The clasts vary from angular to rounded and include a variety of basement granitoid rocks, although most were derived from the Thluicho Lake Group. Centimetre-scale, fine- to medium-grained sandstones are locally interbedded with the conglomerate. The stratigraphically overlying unit varies from orange, thinly-bedded, medium-grained sandstone to a red-cream, fine- to medium-grained variety with scattered pebbles and granules. Both commonly contain low-angle, centimetre- to metre-scale trough cross-stratification and are intercalated with pebble lags deposits exhibiting graded bedding. On Slate Island, horizontally bedded polymictic conglomerate of the basal Martin Group overlies steeply dipping strata of the upper Thluicho Lake Group with angular unconformity (Figure 12). Lengthier descriptions of the Martin Group are provided by Mazimhaka and Hendry (1985) and by Scott (1978), although a number of the latter’s reported showings along the Charlot River were reinterpreted as hematitic fault breccias derived from Thluicho Lake Group strata by Hunter et al. (2003). The set of brown, fine- to medium-grained, essentially unmetamorphosed diabase dykes previously reported in the region (Scott, 1978; Ashton and Hunter, 2003) extends throughout the Camsell Lake area, but their size and numbers diminish west of the Thluicho Lake Group. They range up to 50 m thick and are generally east striking. They intrude the Thluicho Lake Group but were not recognized in the Martin Group. In the Uranium City area, they are thought to have acted as feeders for the Martin Group basalts (Tremblay, 1972; Figure 11 - Polymictic conglomerate of the Martin Group Morelli et al., 2001). A ca. 1.82 Ga age (R. Hartlaub, from 1.5 km south of Charlot River mouth (UTM 604195E, pers. comm., 2003) together with petrologic similarities 6606816N, NAD83); note hematized rims on several clasts.

Saskatchewan Geological Survey 7 Summary of Investigations 2004, Volume 2 suggest that the diabases are correlative with the 1827 ±4 Ma (Bostock and van Breemen, 1992) Sparrow diabase dykes that extend across much of the western Northwest Territories (Ashton et al., 2004). Rare, pink to white, straight-sided, medium-grained to pegmatitic granite dykes that intrude the diabase dykes are thought to represent crustal melts derived during late stages of the Thelon-Taltson and Trans-Hudson orogenies.

g) Athabasca Group White pebbly to cobbly sandstone and minor conglomerate of the Fair Point Formation (Ramaekers, 1990) outcrop south of Camsell Portage on Charlot Island (Figure 2). Most of these horizontally bedded and unmetamorphosed rocks occur as frost-heaved blocks, but rare in situ outcrops together constitute one Figure 12 - Angular unconformity (hand on horizontal of the few exposures of this basal unit of the contact) between steeply dipping strata of the Thluicho Lake northwestern Athabasca Basin. Group and polymictic conglomerate of the Martin Group; from Slate Island (UTM 590000E, 6608000N, NAD83).

4. Structure and Metamorphism Deformational events prior to mylonitization and subsequent deposition of the Thluicho Lake Group are difficult to distinguish due to overprinting by the mylonitic fabric. In adjacent areas to the east, D1 deformation and associated upper amphibolite facies metamorphism produced the gneissosity in the basement rocks, much of which is defined by melt leucosome (Ashton et al., 2001; Ashton and Hunter, 2003). A second event (D2) produced east-southeast– trending isoclinal folding that is best preserved east of Uranium City, whereas a third phase (D3) was responsible for close to tight, northeast-trending regional folds that characterize the Black Bay straight belt. An overprinting axial planar foliation defined by biotite to the east (Boivin, 2002; Ashton and Hunter, 2003) suggests that this amphibolite facies metamorphism either persisted until D3, or was re-attained at that time. A ca. 1.90 Ga metamorphic age obtained from a gabbro dyke 15 km east of the study area is tentatively thought to represent the time of D3 deformation and is attributed to the Thelon-Taltson Orogen (Ashton and Hunter, 2003).

These D1 to D3 events are also thought to have affected the basement rocks in the study area prior to mylonitization, which is considered a late-D3 event. The main mylonitic fabric is axial planar to rare isoclinal folds, which are considered D2 or D3 structures. Mylonitization took place under amphibolite facies conditions, but most sheared rocks in the vicinity of the Thluicho Lake Group have been retrogressed to produce greenschist facies assemblages. Unambiguous kinematic indicators are rarely preserved. The apparent northeasterly trend of the zone is similar to that of the Black Bay Fault and a similar dextral sense of displacement is generally implied (Ashton et al., 2001; Figures 6 and 13), although subsequent deformation renders such indications tenuous.

Post-Thluicho Lake Group folding includes: an isoclinal D4 phase; an east-northeast–trending, open to close D5 phase; and a gentle to open, upright, north- trending D6 phase that also affects the Martin Group. The D4 isoclines were accompanied by greenschist- facies metamorphism (Scott, 1978; Hunter et al., 2003, 2004). In many places, they are recognized solely by reversals in facing direction, whereas elsewhere a strong bedding-parallel axial planar fabric is developed about closely spaced fold closures (Figure 14). Bedding-parallel shear zones suggest that thrust faulting may have accompanied the isoclinal folding. Due to the decrease in competency accompanying mylonitization, abundant D4 and D5 folds are common in the basement rocks as well as the Thluicho Lake Group (Figure 15). D6 folding is particularly well defined by mylonitic psammopelitic to pelitic gneiss- Figure 13 - Dextral shear bands developed in mylonitic rocks of the garnetiferous pelitic migmatite and derived migmatites in the Strike Lakes area (Figure 2), where it anatectic granite unit; from 5.5 km south of Charlot River mouth (UTM 605458E, 6603053N, NAD83).

Saskatchewan Geological Survey 8 Summary of Investigations 2004, Volume 2 is characterized by the strong axial planar fabric defined by pseudomorphed garnet (Figure 10). Brittle and brittle-ductile faulting is widespread and intense. Map-scale offsets are most noticeable in the Thluicho Lake Group due to its stratigraphic references, but basement rocks have been equally affected. Although this style of deformation probably continued episodically following the more ductile, deeper crustal conditions during D3, many of the brittle and brittle-ductile features appear to have formed or been significantly reactivated during and after D6. The east-west–shortening stress regime inferred during that time produced a conjugate set of northeast-striking dextral and southeast-striking sinistral faults (Ashton et al., 2001; Ashton and Hunter, 2003). Accompanying north-south extension led to east-striking normal faults, Figure 14 - Isoclinal D4 folding developed in of the which define much of the northern shoreline of Lake Thluicho Lake Group; from 3 km west of Camsell Portage Athabasca and provided conduits for the east-west (UTM 595293E, 6609090N, NAD83). diabase dykes. Downdropping of the southern blocks along successive faults (Figure 15) helped to preserve remnants of the Thluicho Lake Group, Martin Group, and Athabasca Group in the area in and west of Ellis Bay (Figure 2).

Most of these east-striking faults are marked by strong lineaments, but alteration and brecciation are also common. Faults exposed along the northern shore of Lake Athabasca exhibit extensive limonitic alteration, silicification, sericitization, and chloritization, the latter of which is commonly accompanied by intense fracturing and brecciation. One such breccia that was previously attributed stratigraphic status and termed the ‘Ellis Bay Formation’ (Scott, 1978) is now reinterpreted as tectonic (Hunter et al., this volume).

Much of this late faulting can probably be attributed to a vise-like stress regime that was set up at about Figure 15 - Up-plunge view of D5 folding in ultramylonitic 1.83 Ga by indentation of the Slave Craton into the gneissic granodiorite from 600 m north of Slate Island Rae-Hearne in the northwest and terminal collision of (UTM 590033E, 6608916N, NAD83). Vertical faces created the Sask, Superior, and Rae-Hearne cratons in the by numerous, spaced, south-side-down, east-striking normal Trans-Hudson Orogen to the southeast (Ashton et al., faults. 2004).

A more detailed description of deformation within the Thluicho Lake Group is provided in a companion paper (Hunter et al., this volume).

5. Economic Geology Cu-Pb-Ag±U deposits and showings, some with anomalous values of Co, Ni, Zn, Au, and As, have been known for some time in the Camsell Portage region (e.g., Scott, 1978; Ashton and Hunter, 2003). The mineralization occurs at, or slightly below, the Thluicho Lake Group–basement contact and was not recognized where the sedimentary succession and unconformity have been removed by erosion. Some of the largest and most abundant of the east- trending diabase dykes are also spatially related to the Cu-Pb-Ag mineralization, many at the property scale. These spatial relationships may indicate that the mineralization formed due to redox reactions at the unconformity that were locally facilitated by heat from the diabase dykes. Given the diversity of elements, the potential sources for the metals include the basement, the Thluicho Lake Group, and the diabase. The potential for a fluid system to provide economic mineralization may be linked to the magnitude of the system and thus to the size of the sedimentary basin. Therefore, it is noteworthy that the Thluicho Lake Group was originally thicker, as indicated by the angular unconformity between it and the overlying Martin Group, and more extensive as inferred by the presence of outliers 6 km east of Wellington Lake (Ashton and Hunter, 2003), 18 km west of Slate Island (Harper, 1996), and about 40 km northwest of the study area (unit 7 of Koster, 1961). The

Saskatchewan Geological Survey 9 Summary of Investigations 2004, Volume 2 <1.92 Ga Burntwood Group, located about 90 km southwest of Slate Island along the northern shore of Lake Athabasca (McDonough and McNicoll, 1997), may be yet another outlier of the Thluicho Lake Group. Previous suggestions that the Thluicho Lake Group is correlative with the Nonacho Group, a broadly coeval and similar siliciclastic succession extending over much of the western Rae Province in the Northwest Territories (Aspler and Donaldson, 1985), are supported by their similar sedimentary facies (Hunter et al., this volume) and ca. 1.92 to 1.82 Ga age brackets (Ashton et al., 2004). They probably represent the remnants of a continuous blanket of molasse that resulted from the uplift and erosion of mountains produced during the Thelon-Taltson orogeny. In addition to the Cu-Pb-Ag±U mineralization, the Saskatchewan Mineral Deposit Index lists several occurrences of uranium from the fault-defined shoreline of Lake Athabasca in the Anderson Point and Island Bay areas.

6. Discussion and Conclusions The early geological history of the Camsell Portage area is recorded by two broad rock types for which little is known. The foliated to gneissic granitoids appear to represent a variably migmatized granodiorite-gabbro suite of the kind usually attributed to arc-type complexes; however, this kind of magmatism may have developed at several times prior to the 1.90 Ga upper amphibolite facies metamorphic event. They are tentatively thought to be part of a widespread ca. 2.64 to 2.58 Ga granodioritic to dioritic suite recognized in the Nolan Domain to the north (Figure 1; Van Schmus et al., 1986) and Beaverlodge Domain to the east (Hartlaub et al., 2004). However, they could alternatively be part of: 1) a 3.0 to 3.7 Ga suite inferred from detrital zircon studies of the Murmac Bay Group (Hartlaub et al., 2004); 2) the 2.3 Ga suite of North Shore granite plutons (Hartlaub, 2004); or 3) ca. 2.0 to 1.9 Ga arc rocks related to the Thelon-Taltson Orogen.

The age and origin of the supracrustal gneisses and migmatites are equally unclear. They share some similarities with the Murmac Bay Group, but lack the quartzite-basalt association and form much more extensive units. Given the new Paleoproterozoic age for the Murmac Bay Group (Ashton and Hunter, 2003), these dominantly pelitic rocks could alternatively represent part of the Archean basement.

The leucogranites have near-minimum melt compositions and are attributed to crustal melting during 1.9 Ga metamorphism, although it would not be easy to distinguish such rocks from melts generated during earlier thermotectonic overprints. The main mylonite zone is also thought to have formed during this 1.9 Ga orogenic event, which is tentatively attributed to terrane accretion at the western Rae craton margin. The resulting mountains are thought to have supplied the detritus for the likely correlative Thluicho Lake and Nonacho groups, which were deposited directly on the metamorphosed rocks in a foreland setting. It is unclear whether the first two folding events to affect the Thluicho Lake Group (D4 and D5) result from late deformation related to Thelon-Taltson orogeny or early deformation related to Trans-Hudson orogeny, but by about 1.83 Ga a vise-like stress regime had been set up in the Rae Province by indentation of the Slave Craton to the west and terminal collision of the Superior, Sask, and Rae-Hearne cratons in the latter orogen to the east (Ashton et al., 2004). Results of this shortening regime included deposition of the Martin Group and lower Dubawnt Supergroup (e.g., Rainbird et al., 2003) in structural basins created by widespread trans-tensional faulting, and emplacement of 1.84 to 1.81 Ga lamprophyre dykes in the east and 1.83 to 1.82 Ga diabase dykes in the west (Ashton et al., 2004). Subsequent D6 folding of these rocks is broadly correlative with post-collisional deformation related to Trans-Hudson orogeny.

The Cu-Pb-Ag±U vein mineralization associated with the Thluicho Lake Group and diabase dykes appears to have formed by a similar process to that responsible for the younger ‘vein-type’ uranium mineralization of the Beaverlodge mining camp. Both are interpreted as unconformity deposits resulting from redox reactions between reducing fluids moving through basement rocks and oxidizing basinal fluids. In the Beaverlodge case, the overlying sedimentary succession was the Martin Group, although some deposits may have formed directly below rocks of the subsequently eroded Athabasca Group. In the case of the Cu-Pb-Ag±U mineralization to the west, it was the overlying Thluicho Lake Group that provided the basinal fluids. Whether the veins resulted from the reworking of former sedimentary copper (or similar) mineralization developed in the sedimentary succession or were introduced from other sources remains unclear. Scattered occurrences of uranium mineralization along the fault-defined northern shoreline of Lake Athabasca probably represent a third generation of unconformity deposits, which are related to the Athabasca Supergroup.

7. Acknowledgments The field work this summer was made both enjoyable and efficient by the cheerful and able assistance of B. Knox, C. Ebel, and J. Lesperance. G. Yeo, assisted by A. Nixon, spent a month in our camp while conducting sedimentological and stratigraphic studies on the Thluicho Lake Group. Dr. K. Bethune visited for a few days at the end of July to provide structural guidance for R. Hunter’s M.Sc. thesis study of the stratigraphy and structural history of the Thluicho Lake Group.

Saskatchewan Geological Survey 10 Summary of Investigations 2004, Volume 2 We are again indebted to D. Turner and his SaskPower colleagues at the Wellington Lake Power Station who provided us with a vehicle and gas, facilitating access to the Charlot River system, the Wellington Lake airstrip, and Lake Athabasca. D. Knox of Parkes General Store in Uranium City graciously and efficiently kept us well supplied for yet another year. The original manuscript was improved thanks to constructive reviews by C.D. Card and R.O. Maxeiner.

8. References Ashton, K.E., Boivin, D., and Heggie, G. (2001): Geology of the southern Black Bay Belt, west of Uranium City, Rae Province; in Summary of Investigations 2001, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.2, p50-63. Ashton, K.E. and Card, C.D. (1998): Rae Northeast: A reconnaissance of the Rae Province northeast of Lake Athabasca; in Summary of Investigations 1998, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 98-4, p3-16. Ashton, K.E., Hartlaub, R.P., Heaman, L.M., Morelli, R., Bethune, K.M., and Hunter, R.C. (2004): Paleoproterozoic sedimentary successions of the southern Rae Province: Ages, origins, and correlations; Geol. Assoc. Can./Miner. Assoc. Can., Jt. Annu. Meet., St. Catharines, May 12 to 14, Conference CD-ROM, Abstr. Vol. 29, p434. Ashton, K.E. and Hunter, R.C. (2003): Geology of the LeBlanc-Wellington lakes area, eastern Zemlak Domain, Rae Province (Uranium City Project); in Summary of Investigations 2003, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003-4.2, CD-ROM, Paper A-1, 15p.

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

Aspler, L.B. and Donaldson, J.A. (1985): The Nonacho Basin (Early Proterozoic), Northwest Territories, Canada: Sedimentation and deformation in a strike-slip setting; in Biddle, K.T. and Christie-Blick, N. (eds.), Strike-slip Deformation, Basin Formation, and Sedimentation, Soc. Econ. Paleo. Miner., Spec. Pub. No. 37, p193-209.

Boivin, D.M. (2002): Structural geology of the Fold Lake area, northwestern Saskatchewan; unpubl. B.Sc. thesis, Univ. Regina, 34p.

Bostock, H.H. and van Breemen, O. (1992): The timing of emplacement and distribution of the Sparrow diabase dyke swarm, District of Mackenzie, Northwest Territories; in Radiogenic Age and Isotopic Studies: Report 6, Geol. Surv. Can., Pap. 92-2, p49-55.

Hale, W.E. (1954a): Gulo Lake, Saskatchewan; Geol. Surv. Can., Pap. 54-6, 1:63,360 map with marginal notes.

______(1954b): Black Bay Map-area, Saskatchewan; Geol. Surv. Can., Pap. 53-15, 18p.

______(1955): Forcie Lake Map-area, Saskatchewan; Geol. Surv. Can., Pap. 55-4, 1:63,360 map with marginal notes. Harper, C.T. (1996): Geology of the Maurice Bay Area (NTS 74N-5 and part of 74N-12); Sask. Energy Mines, Rep. 219, 27p. Hartlaub, R.P (1999): New insights into the geology of the Murmac Bay Group, Rae Province, northwest Saskatchewan; in Summary of Investigations 1999, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.2, p17-26. ______(2004): Archean and Proterozoic evolution of the Beaverlodge Belt, Churchill craton, Canada; unpubl. Ph.D. thesis, Univ. Alberta, 189p. Hartlaub, R.P. and Ashton, K.E. (1998): Geological investigations of the Murmac Bay group, Lake Athabasca North Shore Transect; in Summary of Investigations 1998, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 98-4, p17-28.

Saskatchewan Geological Survey 11 Summary of Investigations 2004, Volume 2 Hartlaub, R.P., Heaman, L.M., Ashton, K.E., and Chacko, T. (2004): The Archean Murmac Bay Group: Evidence for a giant Archean rift in the Rae Province, Canada; Precamb. Resear., v131, p345-372. Hunter, R.C., Bethune, K.M., and Ashton, K.E. (2003): Stratigraphic and structural investigations of the Paleoproterozoic Thluicho Lake Group, central Zemlak Domain (Uranium City Project); in Summary of Investigations 2003, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003- 4.2, CD-ROM, Paper A-2, 14p. ______(2004): Stratigraphic and structural investigations of the Thluicho Lake Group, Zemlak Domain, Rae Province; Geol. Assoc. Can./Miner. Assoc. Can., Jt. Annu. Meet., St. Catharines, May 12 to 14, Conference CD-ROM, Abstr. Vol. 29, p434. Koster, F. (1961): The Geology of the Thlainka Lake Area (West Half), Saskatchewan; Sask. Dep. Miner. Resour., Rep. 61, 28p. ______(1967): The Geology of the Harper Lake Area (South Half), Saskatchewan; Sask. Dep. Miner. Resour., Rep. 111, 37p Mazimhaka, P.K. and Hendry, H.E. (1984): The Martin Group, Beaverlodge area; in Summary of Investigations 1984, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 84-4, p53-62. ______(1985): The Martin Group, Charlot Point and Jug Bay areas; in Summary of Investigations 1985, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 85-4, p67-80.

McDonough, M.R. and McNicoll, V.J. (1997): U-Pb age constraints on the timing of deposition of the Waugh Lake and Burntwood (Athabasca) groups, southern Taltson magmatic zone, northeastern Alberta; in Radiogenic Age and Isotopic Studies: Report 10, Geol. Surv. Can., Current Research 1997-F, p101-111.

Morelli, R., Ashton, K.E., and Ansdell, K. (2001): Geochemical investigation of the Martin Group igneous rocks, Beaverlodge Domain, northwestern Saskatchewan; in Summary of Investigations 2001, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.2, p64-75.

Persons, S.S. (1983): U-Pb geochronology of Precambrian rocks in the Beaverlodge area, northwestern Saskatchewan; unpubl. M.Sc. thesis, Univ. Kansas, 68p.

Rainbird, R.H., Hadlari, T., Aspler, L.B., Donaldson, J.A., LeCheminant, A.N., and Peterson, T.D. (2003): Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada; Precamb. Resear., v125, p21-53.

Ramaekers, P. (1990): Geology of the Athabasca Group (Helikian) in northern Saskatchewan; Saskatchewan Geological Survey, Sask. Energy Mines, Rep. 195, 49p.

Scott, B.P. (1978): The Geology of an Area East of Thluicho Lake, Saskatchewan (part of NTS area 74N-11); Sask. Miner. Resour., Rep. 167, 51p.

Tremblay, L.P. (1972): Geology of the Beaverlodge Mining Area, Saskatchewan; Geol. Surv. Can., Mem. 367, 265p. Van Schmus, W.R., Persons, S.S., Macdonald, R., and Sibbald, T.I.I. (1986): Preliminary results from U-Pb zircon geochronology of the Uranium City region, northwest Saskatchewan; in Summary of Investigations 1986, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 86-4, p108-111.

Saskatchewan Geological Survey 12 Summary of Investigations 2004, Volume 2