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

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 Saskatchewan Geological Survey 2 Summary 579000 584000 589000 594000 599000 604000 609000 0 0 Su 0 0 ake Thluicho 0 0 L Forcie LGd 6 6 Lake Gd Cu 1 1 6 6 6 6 L Cu Cu Mx Tp Tc Tp Tc Cu Tc o Tp rlo ate Tp W ke n Tp Cu a o L t g e Zp n k Cu i l Gd U a Gd l L e Cu Cu Tp Tc W 0 0 Zp Gd Gd Cu Tp Zu Zp 0 0 Gd Gd Cu ke Tc 0 0 Zp Tc a Sw Zp Tc w L Tc 1 1 Zp Zp L Gd Lo 1 1 6 6 Zp Zp Sp Tp 6 6 L Gd Tc Sp Tc Zp Gg Tc Cu Zp L Tc Tp U Gd Ellis Bay TpTcTcTp Tp Zp Gd L Tc Tc e Zp Zp Gd Tc Tc Zu k Zp Tp Tc a Gg Gd Tp Cu L Tp L L Gg Tc TcTp Tc Rc Zp Tc k Tc a Rc l Cu TcRc Rc Su mZpg Halfway e Slate Cu Charlot Z Point Island Peninsula 0 0 Rc 0 0 Gd Zpg 0 0 L Afp Charlot 6 6 0 0 Island Su 6 6 Lake Athabasca 6 6 Gd McKenzie Legend Point Afp Zu Contact Athabasca Group Mixed gneiss U Strike Rc Martin Group Zp Pelitic gneiss-migmatite- Unconformity diatexite, minor quartzite Lakes Tc Camsell Portage Formation, Zpg Garnetiferous pelitic migmatite Border of Mylonite Zpg Thluicho Lake Group 0 0 U ofInvestigations 2004,Volume2 Fault/Lineament 0 0 Powder Lake Formation, Gd Gneissic granodiorite Tp U Mx 0 0 1 1 Thluicho Lake Group Gom Granodioritic orthogneiss 0 0 Su 6 6 Mylonite Gg 6 6 Granitic orthogneiss L Garnetiferous leucogranite Island Bay Gom Mx Murmac Bay Group Gom Gon Gon 579000 584000 589000 594000 599000 604000 609000 Figure 2 - Simplified geological map of the Camsell Portage area. 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).

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