The Amer Belt, hosting eleven informal formations of the Amer supergroup, is proposed as type area for four regional Paleoproterozoic sequences (Ps1–Ps4) in central Rae Craton, western Churchill Province. The ca. 1.9–1.865 Ga Snowbird orogeny (DP1) affected only Ps1–Ps3, whereas the ca. 1.87–1.81 Ga Hudsonian orogeny (DP2) affected all four. Sequence Ps1 Ayagaq Lake formation (<2.3 Ga quartzite) initiates as schistose basal polymict orthoconglomerate unconformably overlying paleoweathered Neoarchean rocks. It transitions upward to lower Ps2 Resort Lake formation: gossanous, recessive, graphitic, pyritic-metalliferous phyllite (Re–Os age: 2126 ± 24 Ma). Ps2 conductors beneath the <1.74 Ga Thelon formation have been drilled for unconformity-related uranium. Gradationally overlying siliceous dolomitic marble (Aluminium River formation) completes Ps2. The gradationally overlying lower Ps3 is either foliated calcareous tholeiitic basalt (Five Mile Lake formation) or grey phyllite (Three Lakes formation). These lateral equivalents host strong linear aeromagnetic markers. The Oora Lake formation foliated feldspathic calcareous sandstone gradationally overlies only the Three Lakes formation and underlies the upper Ps3 Showing Lake formation (interbedded phyllite and foliated calcareous arkose) whose two arkose members host laterally continuous disseminated uraninite + magnetite forming linear aeromagnetic markers. Pristine sequence Ps4 molasse, deposited and weakly deformed during DP2, comprises four formations of rhythmically interbedded, deep maroon to pink and green, feldspathic-lithic arenite, conglomerate, and mudstone that unconformably overlie and include clasts of DP1-deformed Ps1–Ps3 + Neoarchean basement. Ps4 detrital zircon (3.05–1.90 Ga) suggests local + distal provenance from south Rae and Slave Cratons, and Taltson–Thelon magmatic zone.
1 Introduction and objectives
In November 2021, the Geological Association of Canada held a symposium entitled “Understanding the Precambrian: a Symposium in honour of Grant M. Young (1932–2020)”. Young’s pioneering mapping and stratigraphic framework for the central Amer Belt (Young 1979) on the central Rae Craton of the western Churchill Province (Figs. 1 and 2) has guided research and exploration for decades, yet was never published. This paper updates and expands the early stratigraphy and mapping of the Amer Belt by Young (1979), Knox (1980), and Patterson (1980a, b, 1981, 1986) with data acquired in 1998, 2004, and from 2006 to the present by the remaining authors.
Fig. 1.
Fig. 1. Geological context of the central Rae Craton, western Churchill Province, northwestern Canada, modified after Rainbird et al. (2010). Projection unknown; approximate scale; coordinates in degrees and minutes.
Fig. 2.
Fig. 2. Extent of the Neoarchean Pukiq Lake formation, the early Paleoproterozoic Amer, Ketyet River, and other comparative belts around and beneath (lighter shades) the Thelon Formation. Location outlined in Fig. 1. The white areas are undivided other Neoarchean to Neoproterozoic rocks. Fault systems are in blue. Line 6–6 shows location of structural cross-section shown in Fig. 6. The map areas of Figs. 8 and 13 are also outlined. Other labels denote localities mentioned in text. Simplified from fig. 3a in Jefferson et al. (in press) and fig. 1a in Tschirhart et al. (2014) which were exported from a geodatabase in NAD_1983_UTM_Zone_14 N; WKID: 26914 Authority: EPSG; Transverse Mercator projection; coordinates in degrees and minutes.
The Geomapping for Energy and Minerals (GEM) uranium project tested the hypothesis that northern Canadian basins have untapped uranium potential that could be revealed by adapting strategies developed in the Athabasca Basin region (Fig. 1) (Jefferson et al. in press). In the Amer Belt, the new GEM data elucidate the geophysical, structural, and stratigraphic contexts of sandstone-hosted (e.g., Gandhi et al. 2015) and other types of uranium occurrences on the central Rae Craton. Tracing graphitic phyllite units within the Amer Belt stratigraphy beneath the Thelon Formation may help locate unconformity-related uranium mineralization (Nutter 1979; Young 1979), analogous to graphitic conductors beneath the Athabasca Supergroup (Hoeve and Sibbald 1978; Jefferson et al. 2007a, b; Alexandre et al. 2009). This study focused on the Amer Belt because of its intrinsic endowment of stratabound sandstone-hosted uranium occurrences in sequence Ps3. The sequences Ps2 and Ps4 conductive pyritic graphitic units have been exploration targets for unconformity-related uranium deposits both under and near the Thelon Formation (e.g., Nutter 1979). Finally, the Amer Belt is the only one with complete representation of the four sequences.
Basement-hosted unconformity-related uranium deposits at intersecting reactivated faults in pyritic metasedimentary rocks of the Woodburn Lake group (Fuchs et al. 1986) may also be found near the Amer Belt, or along fault zones in radiogenic granite (e.g., Potter et al. 2020; Tschirhart et al. 2021). For all of these possible settings, improved structural and stratigraphic knowledge of the Amer Belt and its substrate will better guide exploration for Proterozoic uranium and Archean gold such as the adjacent Amaruq mine (e.g., Valette et al. 2020). In the case of unconformity-related uranium deposits, intersecting arrays of reactivated faults are key exploration targets in both Paleoproterozoic and Archean basement settings (Jefferson et al. 2007a, b,in press).
Better structural and stratigraphic understanding of the Amer and other early Paleoproterozoic belts in the central Rae Craton (Figs. 1 and 2) further refines the four supracrustal assemblages introduced by Rainbird et al. (2010) and updated by Pehrsson et al. (2013b) as sequences Ps1, Ps2, Ps3, and Ps4. This paper proposes new type and reference areas for the four sequences that are completely represented (Fig. 3) only in the contiguous outcropping portion of the Amer grand synform northeast of the Aberdeen Subbasin.
2.1 Structural paradigm for distinguishing Paleoproterozoic sequences
A structural–stratigraphic paradigm for the Ketyet River Belt and inferred to apply to the Amer Belt (Pehrsson et al. 2013b) was verified in the Amer Belt (Fig. 4) by Calhoun et al. (2014). According to this paradigm, sequences Ps1–Ps3 were intensely deformed during DP1 (the ca. 1.9–1.865 Ga Snowbird orogeny), an early accretionary phase of the Hudsonian Orogeny in the Rae Craton (Hoffman 1988; Berman et al. 2007; Corrigan et al. 2009; Pehrsson et al. 2013b, Thiessen et al. 2020; Jefferson et al. in press). The overlapping ca. 1.87–1.80 Ga main phase of the Hudsonian Orogeny (DP2 in the central Rae Craton) further deformed sequences Ps1–Ps3 and was the first event to deform and metamorphose Ps4. The penetrative northeast-striking foliation (SP2) that characterizes much of the Rae Craton is associated with the ca. 1.85–1.82 folding and thrusting component of the Hudsonian orogeny (Davis et al. 2021); incipient and waning effects of the orogeny (e.g., development of Riedel shear arrays, extensional faulting, local granite emplacement, and filling of the Baker Lake Basin) are discussed in Jefferson et al. (in press). DP1 strain is most intense in fine-grained siliciclastic units and siliceous dolomitic marble (Calhoun et al. 2014). In contrast, foliated sandstone units and transitional impure carbonate strata locally show much less internal strain, although those same foliated sandstone units define macroscopic DP1 and DP2 structures. Major contacts between Ps1 through Ps3 formations are strongly foliated due to strain partitioning; strain is heterogeneous fractally from the scale of regional mapping, through bedding in outcrop, to lamination in thin section. In contrast, sequence Ps4 was deposited during early DP2 and was generally deformed only weakly by DP2 and later events. It follows that weakly deformed strata in the Amer Belt which lack DP1 structures belong to Ps4. White et al. (2021, this volume) address the paradox of how this project reaffirmed Young’s (1979) stratigraphic sequence, despite the identification of such complex polyphase deformation.
Fig. 4.
Fig. 4. Structural paradigm distinguishing sequences Ps1–Ps3 from sequence Ps4. (A) Generalized cartoon of DP1 in sequences Ps1–Ps3 and DP2 in all four sequences, after Pehrsson et al. (2013b) and Jefferson et al. (2015). S0 is weakly deformed primary bedding. See text for full explanation. (B) Outcrop of carpet-style isoclinal FP1b, FP1c of translated S0 = SP1a in the Ps1 quartzite at station 10JPC-10 (Fig. 2) (NRCan Photo DB Number 2022-404, by C.W. Jefferson). Inset “IMG_7037” shows tectonically flattened ripple marks still preserved (NRCan Photo DB Number 2022-405, by C.W. Jefferson) on the transposed bedding surface. (C) Line drawing of (B): solid lines are sericite schist partings between isoclinally folded (FP1b, c) tabular quartzite beds that are transposed parallel to foliation SP1a (dashed lines).
The geological and metallogenic data that define the Amer supergroup are extrapolated the length, breadth, and depth of the Amer grand synform by integrating them with geophysical data ((Tschirhart 2013; Tschirhart et al. 2011a, b, 2013, 2014, 2017), Calhoun et al. (2014), and Jefferson et al. (2015, in press)). The most recent Residual Total Field (RTF) data for the Amer Belt (Fig. 5) illustrate key spatial associations between aeromagnetic anomalies and lithostratigraphic units. Integration of detailed gravity and magnetic transects with outcrop data constrains structural cross-sections that in turn provide thickness estimates of stratigraphic units (e.g., Fig. 6). Tschirhart et al. (2014, 2017) also extrapolated the Amer and other supracrustal belts beneath the flat-lying Thelon Formation in the Aberdeen Subbasin (Fig. 2). Further integration of geophysics with satellite imagery, surficial geology and air photographs (Shelat et al. 2012a, b; LaRocque et al. 2012; Anand and Jefferson 2017a, b) helped define folds, faults, and dykes that affected the Amer Belt (e.g., Figs. 7 and 8). Mineral occurrences are treated as specialized rock types described as integral parts of unit descriptions for lithostratigraphic analysis. We assign high importance to mineralogy and mineral occurrences of a stratabound nature, in particular magnetic minerals and uranium.
Fig. 5.
Fig. 5. Geological elements of the Amer Belt region expressed as aeromagnetic anomalies on a shaded relief Residual Total Field (RTF) image (illumination from 090°). Labels are as follows: 3, Three Lakes formation; 5, Five Mile Lake formation; BIF, Archean banded iron-formation; H, Hudson granite; Mac, Mackenzie diabase; McR, McRae Lake diabase; N, Nueltin granite; S, Showing Lake formation; SIs, Snow Island suite deep pluton; U, Archean ultramafic rocks. After Jefferson et al. (in press, Fig. 3D), image leveled and combined from Tschirhart et al. (2011a), and Miles and Oneschuk (2013). The primary RTF image was exported from a GIS in NAD_1983_UTM_Zone_14 N; WKID: 26914 Authority: EPSG; Transverse Mercator projection; coordinates in degrees and minutes.
Fig. 6.
Fig. 6. Northwest (NW)–southeast (SE) structural cross-section along the southwestern edge of the contiguous outcropping Amer and Naujatuuq belts, after Fig. 1 of Tschirhart et al. (2014) and Fig. 6 of Tschirhart et al. (2017). Location of transect shown in Fig. 2 (labelled 6–6), Figs. 5, 8 (in part), and 13 (in part). The upper labels explain differing data sources for three panels. Top centre: dots = measured magnetic and gravity anomalies, solid line = gravity anomaly calculated from 2017 forward model. The gravity-constrained bottom panel differs slightly from the 2017 forward model. Numbers within units denote density in g/cm3. T1 through T4 denote formation-scale subdivisions of the Tahiratuaq group. CT denotes local conglomerate at the T2/T3 boundary.
Fig. 7.
Fig. 7. Example of continuous mapping in areas of sparse outcrop by integrating high-resolution aeromagnetic data with spatially limited legacy outcrop data. Map area is outlined in Fig. 8. Outcrops are coloured according to lithology. (A) Colour-enhanced portion of scanned original 1:50000 scale outcrop map in west-central NTS 66G1 from Young (1979). No outcrops were found in the corners covered by (B, C). (B) Photograph of one of the stromatolite bioherms constituting the biostrome (red in A) (NRCan Photo DB Number 2022-406, by C.W. Jefferson in 1979). (C) Partial representative structure of microbial laminae in (B). (D) LANDSAT image of the same area as (A) showing the same outcrop locations plus the types and distribution of unconsolidated cover materials. (E) Tilt angle image of the high-resolution aeromagnetic data, showing the same outcrop areas and linear markers numbered 1 through 3B. (F) Interpreted geology of the area, integrating all of the above. Dashed lines with tics denoting interpreted dip directions and numbered 1 through 3B denote the traces of linear aeromagnetic anomalies in (E).
Fig. 8.
Fig. 8. Geology and uranium occurrences of the Amer Belt, after Jefferson et al. (2015). Area of this map is outlined in Fig. 2. Rocks older and younger than sequences Ps1–Ps4 are undivided black. Inset map exemplifies the spatial correlation of sandstone-type uranium occurrences with linear aeromagnetic anomalies of the Showing Lake formation. Base for RTF inset map is high-resolution shaded relief (“sun” from 090°) Residual Total Magnetic Field image derived from Tschirhart et al. (2011a). Polygons and aeromagnetic image exported from a GIS in NAD_1983_UTM_Zone_14 N; WKID: 26914 Authority: EPSG; Transverse Mercator projection; coordinates in degrees and minutes.
Calhoun et al. (2014) and White et al. (2021, this volume) demonstrate, with the aid of geophysics, structural mapping that tests and refines the stratigraphy of the highly deformed sequences Ps1 through Ps3 at the northeastern end of the Amer Belt. In the context of the structural–stratigraphic paradigm (Fig. 4), Fig. 7 illustrates the integrated geophysical–geological mapping process for the only weakly deformed and anchizone metamorphic facies sequence Ps4 in the southwestern part of the belt. Figure 7 integrates legacy outcrop data of Young (1979) with LANDSAT and new aeromagnetic data in the area of a Ps4 stromatolite biostrome. Drift cover is extensive and outcrops are sparse (Figs. 7A and 7D). The dominant rock type in this area is pink cross-bedded arkose and maroon desiccation-cracked mudstone. The deep brown to orange weathering stromatolitic dolostone (Fig. 7B) overlies desiccation-cracked maroon calcareous mudstone, and consists of weakly elongate, juxtaposed bioherms, about 50 cm in diameter, with branching and radiating columns 1–5 cm across (Fig. 7C). The sparsely outcropping biostrome is located at linear transitions from low to high aeromagnetic intensity. The aeromagnetic anomalies are clearest on tilt angle images (Fig. 7E). The asymmetry of the linear aeromagnetic anomalies implies bedding dips and fold axes that are consistent with those determined in outcrop (Fig. 7F).
2.3 Canada Uranium Database
The Canada Uranium Database (Gandhi et al. 2015) details uranium occurrences that extend throughout the core of the Amer grand synform (Fig. 8). Spatial analysis of linear aeromagnetic anomalies in this study (Fig. 5; RTF inset to Fig. 8) established the regional lithostratigraphic continuity of the two stratabound, sandstone-hosted uranium zones that Knox (1980) mapped at outcrop scale. Based on that outcrop-scale mapping, we interpret the wide spacing of known stratabound sandstone uranium occurrences as a function of limited exposure due to the extensive heavy drift cover, despite the laterally continuous stratabound uranium distribution. The two linear aeromagnetic highs outlined by high-resolution RTF data guided both detailed mapping of the Showing Lake formation, and metallogenic analysis (see Showing Lake formation below). Calhoun et al. (2014) further corroborated a link between disseminated magnetite in foliated sandstone and the linear aeromagnetic anomalies of the Showing Lake formation.
2.4 Structural–metamorphic vs sedimentological terminology of lithostratigraphic units
Metamorphism associated with the DP1 deformation appears to have been remarkably low, albeit sufficient to facilitate extreme ductile deformation and develop multiple schistosities up to phyllite facies. Metamorphism associated with the DP2 deformation (MP2) ranges from anchizone to lowermost greenschist facies (local muscovite and chlorite) in Ps4 and the southwest, through biotite-grade greenschist facies in the middle of the belt (Knox 1980), to local amphibolite facies in the northeast (Patterson 1980a, 1981, 1986).
DP1 deformation, definitively restricted to Ps3 and older strata, is nevertheless heterogeneous, as is DP2. Descriptions of units thus vary according to their diagnostic field appearances. For most of the Ps1 through Ps3 lithostratigraphic units, this results in rock names such as quartzite, graphitic phyllite, schist, mafic schist, foliated basalt, phyllitic siltstone, foliated sandstone, and dolomitic marble. In contrast, even the fine-grained strata in sequence Ps4 preserves delicate primary sedimentary structures and it has only open to kinked DP2 folds, hence the use of sedimentary rock terms throughout.
3 Lithostratigraphy and depositional environments of the Amer Belt
3.1 Definition of the Amer Belt
The Amer Belt (Figs. 1, 2, and 3) is a single, contiguous 275 km long, grand synform with multiple internal structures. It extends in outcrop for about 125 km northeast of the Aberdeen Subbasin (Tella 1994), continues in the subsurface for about 120 km beneath the Thelon Formation (Tschirhart et al. 2014, 2017) and forms a number of inliers within the 1.75 Ga Pitz Formation (Wharton Group, inset to Fig. 1; see section: “The Late Paleoproterozoic Dubawnt Supergroup and associated igneous rocks” below) about 30 km southwest of Beverly Lake (LeCheminant et al. 1984). The Amer grand synform includes Neoarchean granitoid and metavolcanic rocks that wrap beneath and are structurally intercalated with the the lower three sequences (Ps1-Ps3) of the Amer supergroup. Subcomponents of the Amer Belt and related early Paleoproterozoic belts in the central Rae Craton are shown in Fig. 2. The Amer mylonite zone, a long-lived ductile to brittle dextral structure that affected early to late Paleoproterozoic rocks (Jefferson et al. in press), delimits the northwest side of the Amer Belt, and trends through its westward transition into the contiguous but poorly known Naujatuuq Belt. The Neoarchean Rumble Belt and North Meadowbank River deformation zone separate the southeast side of the Amer Belt from outliers of the Ketyet River Belt. More distant Paleoproterozoic belts compared below are shown in Fig. 1. These belts form part of the Archean to early Paleoproterozoic structural–metamorphic complex upon which the Dubawnt Supergroup rests.
3.2 Archean
The Neoarchean Woodburn Lake group (Pehrsson et al. 2013b) and undefined supracrustal remnants in granulite facies terrane constitute about 15% of the map area (Fig. 2). The 2.75 to <2.6 Ga Woodburn Lake group is a tectonic collage of seven supracrustal assemblages, including submarine to subaerial volcanic rocks, ranging from komatiite to rhyolite, metagreywacke with sparse thin beds quartzite and carbonate, and extensive iron-formation, hosted by epiclastic rocks and metagreywacke (Jefferson et al. in press).
Voluminous bimodal granitoid and extrusive rocks of the 2.63–2.58 Ga Snow Island suite (SIs) form most of the remaining Archean basement (Figs. 3 and 6), interpreted as products of a westward-directed continental arc that spanned the Rae Craton (Peterson et al. 2015b, in press). The extrusive component of the SIs (Pukiq Lake formation, Figs. 2 and 3) is discontinuously preserved immediately beneath schistose conglomerate and quartzite of the Ps1 sequence (below). Clasts of both previously foliated granite and rhyolite porphyry are locally abundant in all three Paleoproterozoic conglomerate units described below.
3.3 Four early Paleoproterozoic Rae Craton sequences: Ps1, Ps2, Ps3, and Ps4
The Rae family of cratons lacks the distinctive early Paleoproterozoic depositional and glaciogenic record characteristic of Superia (the Huronian Supergroup on the southern margin of Canada’s Superior Province) and Vaalbara (age-equivalent glaciogenic strata on the Kaapvaal and Pilbara Cratons of southern Africa) (Pehrsson et al. 2013a). There is no sedimentary, structural, or metamorphic record in the central Rae Craton between the 2.58 Ga SIs and <2.2 Ga Ps1 (“Gap” in Fig. 3). There is only detrital zircon evidence in this region for the 2.35 Ga Arrowsmith orogeny described by Berman et al. (2013), and those detrital zircon occur only in Ps4 (Jefferson et al. in press).
Informal formation names for map units of the Ps1 through Ps3 sequences from Young (1979) are maintained here, pending formal naming of appropriate topographic features. All of the lakes used as reference names are informal, hence the lower case “l” for these lakes on the figures. When combined as a formation name however, the upper case “L” is used for these informal lakes. The new but as-yet informal group names Qikiqtarjualik (comprising formations of the Ps1–Ps3 sequences) and Tahiratuaq (made up of formations in sequence Ps4, formerly the Itza Lake formation of Young 1979) are the formalized Inuktitut names for lakes close to their respective type areas, taken from Pelly and Nales (1986). For simplicity in this paper we use the sequence codes Ps1 through Ps4. The codes T1 through T4 denote formation-level map units of the Amer sequence Ps4.
3.3.1 Sequence Ps1: Ayagaq Lake formation
The Ayagaq Lake formation name is derived from the 3.2 km long, northerly elongate, informally named Ayagaq lake in the south corner of the belt (Figs. 8 and 13). The pointed shape of this lake resembles the handle of a traditional Inuit toy called an Ayagaq. The Ps1 sequence comprises schistose basal conglomerate, a highly weathering-resistant unit of vitreous highly deformed orthoquartzite, here simply termed quartzite, and an upper moderately resistant foliated arkosic–lithic and carbonaceous transition unit with local schistose conglomerate. Most of the quartzite has a massive, metamorphosed aspect. Bedding, where recognizable, shows evidence of multiple deformations. Ancestral Inuit quarried the Amer Belt quartzite for spear points and cutting implements (Baker Lake Elders Committee). The type locality is east of Amer Lake on the southeast side of the belt (Fig. 8) where the quartzite forms the outer limbs of the Fifty Mile syncline (Fig. 9). The original type locality of Young (1979) at the south corner of the exposed quartzite remains a key reference area because it contains the Ayagaq lake itself, excellent exposure of the contact between the basement SIs granite and the basal Ps1 conglomerate, plus the overlying red-bed member and much of the vitreous quartzite member, however it is not suitable as a type section because its upper contact with sequence Ps2 is either unexposed or faulted (Figs. 10E and 10F).
Fig. 9.
Fig. 9. Geological transect east of upper Amer Lake (location shown in, and projection the same as Fig. 8). (A) Type localities for the structural paradigm (Fig. 4, station 10JPC-10), and sequences Ps1 (Ayagaq Lake formation), upper Ps2 (Aluminium River formation), and lower Ps3 (Five Mile Lake basalt formation) in the Amer Belt. (B) Cross-section northwest–southeast interpreted from outcrop data shown in (A) and from integrated gravity and magnetic data (Fig. 5, Tschirhart 2013; Calhoun et al. 2014).
Fig. 10.
Fig. 10. Sequence Ps1, the Ayagaq Lake formation. (A) Maroon basal schistose orthoconglomerate and pink pebbly arkosic quartzite on the southeast side of the Ayagaq Lake formation reference area (Fig. 8). Hammer is about 45 cm long (NRCan Photo DB Number 2022-407, by C.W. Jefferson 1979). (B) Sericite schist partings in pale green vitreous quartzite (parallel to felt pen) define transposed bedding (S0 = SP1) and range from stylolitic to structurally lineated and shingled in the R22 area (Fig. 8). Apparently opposing festoon cross-beds are highlighted in cross-section by late hematite alteration but not visible on the sheared S0 = SP1 surfaces. Pink hematite at bottom is a late thin coating along a joint plane (NRCan Photo DB Number 2022-408, by C.W. Jefferson in 2008). (C) Shingling and mineral lineation on a transposed bedding plane parting (SP1a) in pale green vitreous quartzite also in the R22 area (Fig. 8) (NRCan Photo DB Number 2022-409, by C.W. Jefferson in 2007). (D) Outcrop view looking south over arcuate tabular bedding in the Ayagaq Lake formation. Blue arrow points to location in (E) (NRCan Photo DB Number 2022-410, by G.M. Young in 1979). (E) Distinct tabular bedding (transposed S0 = SP1) in Ps1 quartzite exposed by subglacial fluvial scour defines an open, kilometre-scale FP4 synform–antiform pair localized between two dextral faults on the northwest side of the Ayagaq Lake formation reference area (oblique aerial perspective indicated by blue “E” in (F)) (NRCan Photo DB Number 2022-411, by G.M. Young in 1979). (F) Surficial and bedrock geology along part of an elongate glacial–fluvial washout. Base is from 1:50 000 scale air photograph NAPL A1504-41; outlined as rectangle “Fig. 10F” in Figs. 8 and 13; exported from a GIS in NAD_1983_UTM_Zone_14 N; WKID: 26914 Authority: EPSG; Transverse Mercator projection; coordinates in degrees and minutes.
The basal Ps1 schistose conglomerate (Fig. 10A) has a texturally seriate, polymict, intact primary framework. Recognizable clast types include vein quartz, Pukiq Lake formation rhyolite, SIs granite, iron-formation, greywacke and mafic volcanic rocks. In the Ayagaq Lake formation reference area, southwestern Amer Belt, the strata are weakly deformed. The colour of the matrix and many lithic pebbles in the upper part of the orthoconglomerate is dark maroon (Fig. 10A), marking the base of the red-bed member described below.
Multiple exposures in the Ayagaq Lake formation reference area show a sharp contact between the weakly to moderately foliated orthoconglomerate and the smooth paleosurface of the bleached and muscovite–schistose underlying SIs granite. The gradation of the bleaching and muscovite content down into fresh pink foliated granite suggest that the basal contact is a metamorphosed paleoweathered zone. Some rusty weathering, pyritic jasper– and quartz–granule conglomerate beds in the overlying quartzite gave McPhar TV-1 scintillometer readings of about 100–700 cps (Young 1979; no laboratory analyses), possibly due to paleoplacer concentrations of heavy minerals such as pyrite, zircon, and monazite.
In the northeastern portion of the Amer Belt, more intense metamorphism and deformation along with heavy lichen cover have in most places disguised the polymict conglomerate as a uniform appearing grey schist with sparse round clasts of vein quartz that impart a paraconglomerate appearance. However, petrography and rare lichen-free surfaces document that the schist is actually a poorly sorted, polymict orthoconglomerate with a seriate intact framework dominated by a wide range of flattened lithic clast types. The primary compositions of the lithic clasts are similar to those of the less-deformed southwestern outcrops, especially the quartz–orthoclase–porphyritic rhyodacite derived from the Pukiq Lake formation, granitoid cobbles from the Snow Island suite and a few distinctive pebbles of iron-formation. In the series of basement-involved imbricate thrust panels on the north side of the Amer Belt (Fig. 8) where the Archean substrate is metagreywacke with iron-formation, the schistose metagreywacke grades upward into muscovite schist which in turn grades upward into feldspathic quartzite, in turn grading upward into vitreous quartzite. Within the schist, several centimetres of strongly foliated lithic pebble conglomerate were identified. The several metres of schist above and below the conglomerate are essentially the same in field appearance. The conglomerate is interpreted as the basal Ps1 conglomerate, the schist below it is thought to be paleoweathered metagreywacke, and the schist above to be transported local regolith. All were metamorphosed to upper greenschist facies schist.
Overall in the basal conglomerate, the dominance of locally derived polylithic clasts and the lateral grain-size and thickness variations suggest rapid deposition in an alluvial setting, influenced by basin-controlling boundary faults oriented parallel to the axis of the belt. Greater grain sizes and thicknesses along the southeastern side of the Amer Belt indicate a more proximal setting compared to the northwest side.
Throughout most of the belt, there is a transitional red-bed member between the basal conglomerate and the vitreous quartzite. The best expression of this red-bed member is in the Ayagaq Lake formation reference area at the south corner of the belt (Fig. 8) where the metamorphic grade and deformation are lower. There, the upper dark maroon conglomerate (Fig. 10A) is interbedded with dark maroon argillite. The first overlying sandstone beds are pink feldspathic quartzite. The maroon to pink colours reflect disseminated finely crystalline hematite in the rock matrix and is distinct from the thin, late hematite coatings that occur along fractures throughout the Ayagaq Lake formation (e.g., Fig. 10B). Elsewhere in the belt, such as the southeast side of the Ifo antiform and in the imbricate basement thrusts along the north side of the belt (Fig. 8), the basal conglomerate grades upward to pink feldspathic to sericitic quartzite, and the maroon argillite is not in evidence. Implications of this stratabound red unit are discussed under the heading “Age controls and provenance of sequences Ps1 through Ps4” below.
Massive appearing white quartzite weathers prominently and forms the bulk of the Ps1 unit. If the quartzite is not exposed, it is interpreted as being absent—even under surficial deposits. The quartzite forms longitudinally continuous rounded ridges that range from 50 m to more than 10 km across and frame the Amer Belt. Cryptic bedding is stylolitized and transposed parallel to SP1 schistosity in most places. In the Ayagaq Lake formation reference area the lower part of the vitreous quartzite is exceptionally well preserved, including decametre-scale trough cross-beds (Young 1979).
An upper unit of pale green vitreous quartzite in the western part of the belt (Knox 1980) petrographically resembles the white quartzite, but has distinct 10–50 cm scale schistose partings along bedding planes (S0) transposed subparallel to SP1 (Figs. 10B–10D). The transposed bedding and the quartzite ridges as a whole define multiple fold generations, from isoclinal FP1 (Figs. 4 and 9), through map scale open FP2 (Figs. 6 and 7) to kilometre-scale FP4 structures associated with strike-slip faults (Figs. 10D–10F).
Thin sections from the southwestern part of the belt show a granoblastic mosaic of quartz grains about 0.15–0.35 mm across and about 1% tiny muscovite grains (Knox 1980). At the northeastern end of the belt (Patterson 1981), chromium green fuchsite is either concentrated in lenses or diffuse in beds, along with very fine opaque minerals. The recrystallized quartz grains have lobate to polygonal sutured boundaries, with most grains elongate parallel to and defining the foliation. Cathodolescence suggests that the primary detrital quartz grain size was seriate, from silt to coarse sand.
The laterally extensive tabular beds with locally recognizable centimetre- to decimetre-scale ripple marks (insets to Figs. 4B and 4C) and decimetre-scale festoon cross-beds (Fig. 10B) suggest shallow marine conditions, although in most places mineral lineation (Fig. 10C), schistosity, and stylolites obliterate bedding plane views. The very large cross-beds in the southwest suggest local aeolian conditions. Locally unimodal cross-beds (Young 1979) suggest fluvial conditions or persistent longshore currents in the lower part of the formation in the Ayagaq Lake formation reference area, assuming no FP1 folds at the measured sites. The quartz-dominant composition may reflect intense chemical weathering of source materials plus extensive winnowing in a broad shelf environment.
3.3.2 Amer sequence Ps1–Ps2 transition zone
Throughout the Amer Belt, the transition zone between Ps1 and Ps2 is marked by a linear gossan. This is bright red and continuous around upper Amer Lake and at the R22 occurrence. At all exposures of the transition, the graphite and pyrite content increase upward over 1–10 m, and the arenaceous component decreases until black phyllite typical of Ps2 is the dominant lithofacies.
Lateral variations in the uppermost Ps1 and at the transition are also present. In the northeastern Amer Belt, the upper part of Ps1 along the south side of the belt includes half-metre thick interbeds of rusty weathering schistose quartz–pebble orthoconglomerate. Two occurrences of such schistose conglomerate are weakly radioactive, as described by Gandhi et al. (2015). Occurrence dp402538, located southwest of Five Mile lake (Fig. 8), has field-identified weakly anomalous radioactivity, listed simply as “U” with no reported laboratory analyses. Occurrence dp402543 southwest of upper Amer Lake (Fig. 8) gave laboratory results of 570 ppm Th, 4.7 ppm U, and trace Au. Such signatures in conglomerate are characteristic of fluvial paleoplacer heavy mineral concentrations such as monazite and zircon, and the rustiness reflects the presence of stratabound pyrite in the conglomerate matrix. The data on these beds in the Amer Belt are solely from Young (1979) and Gandhi et al. (2015), the latter having proposed a paleoplacer origin. The present authors did analyze petrography and geochronology on a similar rusty conglomerate at the same transitional position in the Schultz Lake horst of the Ketyet River Belt (sample 08JP66D1, detrital zircon results reported by Davis 2021). There the pyrite, monazite, and zircon grains are similar in grain size, there is no uraninite and Th > U, however, we did not analyze textures of the pyrite to further test the detrital idea. Given the subaerial depositional environment indicated by fluvial sedimentary structures, the abundance of pyrite and the lack of red beds at the top of Ps1 suggest that the atmospheric oxygen level at that time was not high and/or deposition and burial were rapid. On the other hand, the red-bed unit near the base of Ps1 suggests there was sufficient oxygen, therefore the controlling factor was likely the slow rate of deposition and burial that provided sufficient time to generate and preserve evidence of that oxidizing diagenetic environment.
At the north end of the type transect (Fig. 9) and at the R22 prospect (Fig. 8), the transition zone is argillaceous and feldspathic, pyritic, and thinly laminated as part of a metre-scale upward transition into rusty weathering graphitic phyllite of the Resort Lake formation. At station 04RAT08 (northeast corner of the Amer Belt, Fig. 8), black graphitic slate forms the partings between the uppermost tabular beds of laminated fine-grained Ps1 quartzite. At station 10JPC-212 near the middle of the Amer Belt (Fig. 8), the schistose partings appear to be maroon, as in places at the southwest end of the belt (Young 1979) (these maroon partings may represent post-metamorphism oxygenation). Along the edge of quartzite outcrop forming the Medial uplift zone (Fig. 8) northeast of Amer Lake, a rusty zone of frost boils includes blocks of massive siliceous pyrite among abundant lenticular chips of graphitic phyllite. Much of the pyrite has been dissolved by weathering, leaving blocks of siliceous sinter with a box-work texture (MacIsaac 2011).
The depositional environment changed rapidly during the Ps1–Ps2 transition. Nearing the top of Ps1, local heavy mineral-rich schistose conglomerate beds record renewed uplift in the source regions. These relatively coarse-grained facies grade rapidly upward to foliated lithic–feldspathic fine sandstone and siltstone into very recessive graphitic phyllite. In localities lacking schistose conglomerate, regional scale deepening is presaged by the development of thin graded foliated quartzarenite beds separated by graphitic mudstone, interpreted as A–E turbidite couplets. The lateral variations in the transitional facies indicate that uplift was differential, likely a result of faulting. The ultimate depth change was extreme, given the new euxinic environment in which iron sulfide minerals and carbonaceous material accumulated along with silt- and mud-sized siliciclastic detritus.
The Ps2 sequence comprises two formation-ranked units: a lower black graphitic phyllite and an upper siliceous dolomitic marble. Both have gradational and intercalated lower and upper contacts. Detailed outcrop studies and structural analysis suggest that the intercalations and thickness changes are both depositional and structural (e.g., Figs. 9A and 9B). Some thickness variations relate to dextral faults and FP4 folds at the Ps1–Ps2 transition (e.g., Figs. 10D–10F).
3.3.3.1 Resort Lake formation: graphitic phyllite and local foliated feldspathic sandstone
The Resort Lake formation marks the base of sequence Ps2 with highly strained grey to black conductive graphitic pyritic slate to phyllite and siltstone (Fig. 11A) with submillimetric laminations transposed parallel to SP1. Rarely exposed in solid outcrop, the phyllite is traced in the northeast end of the belt as linear zones of frost-boiled muddy till containing abundant lenticular flakes of phyllite (Fig. 11B) between the Ps1 quartzite and the upper Ps2 carbonate unit. Exposure of any kind is rare in the southwestern part of the Amer Belt, where Knox (1980) only noted “impure clastic rocks” at the unexposed transition between the quartzite and carbonate units. The phyllite at the northeastern end of the Amer Belt is made up almost entirely of muscovite, chlorite, and quartz with lesser amounts of anhedral titanite and rutile (MacIsaac 2011). A member of foliated pyritic lithic arkosic argillaceous sandstone (unit 8 of Patterson 1980b, 1981) is thickest and most extensive in the central northeast, where it weathers light brown to rusty white and grades upward into the Aluminium River formation dolomitic marble. Around the northeast oval (Fig. 8), the foliated sandstone forms gentle linear mounds. At the base of the type transect, it is interbedded with dark grey phyllite.
Fig. 11.
Fig. 11. Examples of sequences Ps2 and Ps3 basalt in the Amer Belt. (A) Graphitic pyritic Resort Lake formation at about 86 m depth in Rad08-13 core south of the R22 uranium occurrence of Titan Uranium Corp. (red “R22” in Fig. 8) (NRCan Photo DB Number 2022-412, by C.W. Jefferson in 2010). (B) Exceptional outcrop (4.5 m bluff) of the Resort Lake formation (lower sequence Ps2) at north side of type transect (* in Fig. 8). Frost-heaved flakes such as in foreground are typically the only surface expression. Arctic hare right of centre (NRCan Photo DB Number 2022-413, by G.M. Young in 1979). (C) Gradational and interbedded upward transition from dark calcareous phyllitic argillite of the upper Resort Lake formation through silty brown argillaceous concretionary dolostone (not marble at this locality) of the lower Aluminium River formation, also at the “*” of the type Resort lake transect, view toward 160° (NRCan Photo DB Number 2022-414, by R. Rainbird in 2004). (D) Aluminium River formation (upper sequence Ps2) in northeastern Amer Belt: grey dolomitic marble with structurally elongated white crystalline quartz lenticles to semicircular bodies, station 04RAT-09 (Fig. 8) in northeastern Amer Belt (NRCan Photo DB Number 2022-415, by R. Rainbird in 2004). (E) Detail of interlayered basaltic tuff (lower Ps3) and dolomitic marble (upper Ps2) at the type locality of the Five Mile Lake formation (Station 09JP013B, Fig. 9), view toward 045° (NRCan Photo DB Number 2022-416, by C.W. Jefferson in 2009). (F) Detail of pink-weathering plagioclase phenocrysts in foliated basalt just north of E (NRCan Photo DB Number 2022-417, by C.W. Jefferson in 2010).
Strong conductors drilled at the R22 prospect (Fig. 11A) and DPR-10 (Fig. 2), the linear gossan and phyllite flakes in frost boils track the Resort Lake formation throughout the Amer Belt and beneath the Thelon Formation. The most carbonaceous and pyritic samples from drill core DPR-10 (Appendix B) are metalliferous, based on thresholds suggested by Huyck (1990). Elements considered metalliferous in one or more samples include Ag, As, Au, C, Cu, Mo, Ni, Pb, and V. Elements characteristic of metalliferous shales that are not elevated in DPR-10 include Co, Cr, Se, P (P2O5), and Zn. One sample (98RAT-4) is elevated in four elements not characteristic of metalliferous shale: Ba, Sr, W, and Th.
The type transect of the Resort Lake formation (Fig. 9) is in the Fifty Mile syncline east of Amer Lake. This transect exposes the context of the black phyllite between quartzite and dolomitic marble on the southeast limb, and between quartzite and basalt on the northwest limb. Reference areas include exceptional exposures at Resort lake (* in Fig. 8), drill core of the R22 uranium occurrence (Fig. 8), and DPR-10 core from beneath the Thelon Formation (Fig. 2).
The Resort Lake formation records deep, quiet, and stratified euxinic subaqueous deposition, except for the laminated lithic feldspathic sandstone members which have received little attention except in terms of their mapped distribution, but may represent shallow subaqueous to emergent depositional environments generated by tectonic uplift and/or lower sea level. The gradational upper contact of the Resort Lake formation through dolomitic siltstone into dolomitic marble of the Aluminium River formation is interpreted as a result of regional regression and reduced clastic influx.
3.3.3.2 Aluminium River formation: siliceous dolomitic marble
Highly strained siliceous dolomitic marble of the Aluminium River formation (Figs. 11C and 11D) forms the upper part of sequence Ps2. The pale grey to tan dolomitic marble consists of saccharoidal calcite, dolomite, and quartz (Knox 1980), is grey to tan weathering, and has no primary sedimentary features except for local fine planar and small trough cross-laminae in the lower and upper transitional dolomitic siltstone members. The saccharoidal to drusy quartz forms irregular laminae, to semicircular nodules or boudins.
Metamorphism ranges from anchizone to lower greenschist facies in the southwest, to upper amphibolite in the northeast. In the northeast, tremolite forms radiating crystals at the interface between quartz and dolomite. Diopside + talc (Tippet and Heywood 1978) and kyanite + sillimanite (Patterson 1986) are present in the extreme northeastern parts of the belt. In the least metamorphosed southwestern Amer Belt the quartz is saccharoidal, the dolomitic marble finely crystalline with very fine transposed and isoclinally folded laminae, but no metamorphic minerals are present (Knox 1980; this study). At both high and low metamorphic grades, rootless folded and refolded FP1b and FP1c isoclines in bedding transposed parallel to SP1a are reconfigured by open FP2 structures (MacIsaac 2011; Calhoun et al. 2014). The complex fold patterns locally mimic stromatolites (e.g., fig. 4 in Patterson 1981).
The base of the dolomitic marble is gradational from black phyllite or foliated sandstone of the Resort Lake formation (Fig. 11C), with carbonate-dominated portions being included in the Aluminium River formation. The top is similarly gradational and interfingered with either mafic schist of the Five Mile Lake formation or grey phyllite of the Three Lakes formation. The type locality (Fig. 9A) is east of Amer Lake (Fig. 8). Reference sites include the Aluminium river area and around station RAT04-9 (Fig. 8).
Assuming that carbonate deposition was biologically mitigated and at a depth similar to the present, the Aluminium River formation records regional shallowing of the marine environment. Discontinuities and/or changing thicknesses of the dolomitic marble unit across folds such as at the type transect (Figs. 9A and 9B) are interpreted as mainly depositional for two main reasons. First, the strain at the stratigraphic level where the carbonate night have been is actually less than the strain where the marble is present. Second, the change from south to north across the Fifty Mile syncline is consistent along the entire length of the Amer Belt. Given this interpretation, the facies change is similar to that observed in Devonian carbonate reefs that thicken across Laramide folds and thrusts in northeastern BC (Taylor et al. 1979) and Devonian carbonate has a similar strike length to that of the Aluminium River formation on the southeast side of the Fifty Mile syncline in the Amer Belt. In the Cordillera, the Devonian biostromes initiated on the upthrown edges of syndepositional fault blocks and thicken basinward until they terminate abruptly, suggesting that paleowater depths became too great for carbonate deposition to initiate.
3.3.4 Amer sequence Ps3: foliated basalt and fine siliciclastic strata with sandstone type uranium occurrences
3.3.4.1 Five Mile Lake formation: basalt
Foliated basalt of the Five Mile Lake formation is one of two units that mark the beginning of Amer sequence Ps3. Sporadic eruption of mafic lapilli tuff of the Five Mile Lake basalt during carbonate accumulation is represented by local gradational intercalations of calcareous mafic schist within the upper marble on the southeast side of the Fifty Mile syncline (Figs. 9 and 11E). The lowest mafic schist defines the base of the formation. On the north side of the syncline (Figs. 9A and 9B) the mafic schist is very thin and gradationally overlies brown lithic sandstone to black phyllite of the Resort Lake formation, with no carbonate present in that limb along the length of the syncline. Most basalt exposures are moderately foliated, massive, metre-scale flows with centimetre-scale, swallowtail to tabular plagioclase phenocrysts in an aphanitic matrix (Fig. 11F). Calcite-chlorite-filled, quartz-rimmed amygdales are locally abundant. In the Ifo lake area (Figs. 6 and 8), a >10 m thick massive gabbro layer with columnar joints, sparse 1–2 cm diameter plagioclase phenocrysts and a 20 cm thick basal chill, but lacking amygdales, overlies silty dolomitic marble with a 1–2 m thick zone of contact-metamorphic tremolite. The unbroken thickness of this gabbro and the lack of amygdales and carbonate alteration contrast with the metre-scale quartz- + carbonate-filled and highly carbonate-altered amygdaloidal flows in the Fifty Mile syncline, thus it is tentatively interpreted as a sill, even though it may be an unusually thick flow whose upper contact is eroded.
The foliated basalt is preserved only as the youngest unit in the cores of FP1 isoclinal synclines. The Fifty Mile syncline (Fig. 9) is the type example wherein the foliated basalt forms the core of the syncline and closes at about 1200 m depth. DP2 folding strongly affected this syncline. Originally recumbent, it dips steeply to the southeast at Five Mile lake and at the type transect, but steeply southwest on the west side of upper Amer Lake where more obvious refolding by tight FP2 antiforms and synforms created Type 2 (Ramsay 1967) zigzag, boomerang, and mushroom interference fold patterns. Throughout this syncline and all other exposures in the Amer Belt, the Ps3 foliated basalt is stratigraphically separate from the Ps1 quartzite, being everywhere separated from it by Ps2 black phyllite, contrary to the intercalated interpretation of Rainbird et al. (2010) and Pehrsson et al. (2013b). Map widths in the Amer Belt are from about 100 to 1000 m in a series of linear exposures in the Fifty Mile syncline (Figs. 8 and 9), with a combined strike length of about 160 km. A few additional exposures extend from 3 to 5 km in strike length, also in the cores of FP1 synclines, are in the south-central and northwestern parts of the Amer Belt. The foliated basalt forms a strong linear aeromagnetic marker (Fig. 5), enabling remote predictive mapping beneath drift and the Thelon Formation (Figs. 5, 6, and 8). It has a unimodal tholeiitic continental basalt composition, with no associated felsic or ultramafic phases, and is both geochemically and structurally distinct from nearby Neoarchean foliated basalt (Patterson et al. 2012).
Basalt, one of the least deformed units that underwent DP1, is nevertheless penetratively foliated. The underlying mafic schist, carbonate and phyllite are interpreted to have accommodated most of the DP1 + DP2 strain. Its calcareous composition and intercalation with marble and phyllite suggest initial subaqueous deposition. The tabular amygdaloidal and vesicular flows lack pillows, suggesting subaerial eruption, although the intense calcite–chlorite alteration suggests interaction with seawater. The extensive longitudinal preservation of foliated basalt in several trends, but gaps elsewhere, are interpreted as a result of eruption localized along northeast-trending fault conduits. Some of these faults may have been active during Ps1 and Ps2 sedimentation.
3.3.4.2 Three Lakes formation: foliated grey phyllitic siltstone
The Three Lakes formation (Fig. 12A) has been called the “mudstone unit” (Knox 1980), “meta-argillite” (Young 1979), or part of the “impure clastics” (Patterson 1986); here generalized as phyllitic siltstone. The Three Lakes formation crops out poorly, except in areas washed clean by glacial meltwater, such as its type transect west of Showing Lake and at the aeromagnetic calibration locality, 10JPC165, in the Northeast oval (Fig. 8). The calibration locality exposes intense isoclinal folds with extremely long limbs, FP2 open refolds, as well as the source of a strong linear aeromagnetic marker (Fig. 12B) (Calhoun et al. 2014). Knox (1980) mapped the type transect in detail (* in Fig. 8). Although graded laminae appear to be primary, they have diffuse margins, reflecting transposition and smearing out parallel to SP1. Laminae with opposing gradations also outline truncated isoclinal folds (FP1 in Fig. 12A).
Fig. 12.
Fig. 12. Examples of upper Ps3 in the Amer Belt. (A) Three Lakes formation at station 10JP166 along its type transect on the west side of Showing lake (“S” near the centre of Fig. 8). Wavy transposed layering in fine and coarse phyllitic siltstone is parallel to SP1b foliation and the axial planes of isoclines (FP1 arrows) with truncated limbs. Dark spots are chlorite-altered MP2 biotite porphyroblasts (Knox 1980) (NRCan Photo DB Number 2022-418, by C.W. Jefferson in 2010). (B) Disseminated euhedral magnetite (m) cuts foliations SP1 and SP2 in the magnetic marker member of the phyllitic Three Lakes formation siltstone at station 10JPC165 (reference outcrop east of Amer Lake, Fig. 8). After Calhoun et al. (2014) (NRCan Photo DB Number 2022-419, by L. Calhoun in 2011). (C) Upper dolomitic sandstone marker of the Oora Lake formation at station 09JP026B (northeastern end of belt, Fig. 8), 540 m northeast of and structurally underlying the PYT uranium occurrence. Line drawing highlights sedimentary structures discussed in text. X = cross-beds, S = possible microbial laminae (NRCan Photo DB Number 2022-420, by Y. Shelat in 2009). (D) Intercalated pink calcareous arkose and grey green phyllite of the Showing Lake formation in northeast Amer Belt (station 08JP068, Fig. 8). Dashed lines SP2 trace DP2 crenulation cleavage refracted from sandstone to phyllite. M = DP2 mullions at the sandstone-phyllite interface. SP2ps denotes pressure-solution layering restricted to the phyllite (NRCan Photo DB Number 2022-421, by C.W. Jefferson in 2008).
The linear aeromagnetic anomaly that marks the Three Lakes formation was first recognized by Knox (1980) on a regional aeromagnetic map (Geological Survey of Canada 1974). It is as strong as that of the Five Mile Lake basalt (Tschirhart et al. 2013, 2017), and is a defining attribute of the upper part of the Three Lakes formation. It is clearly stratabound but associated with disseminated euhedral magnetite that transects SP1 and SP2 in finely crystalline phyllite that lacks any sand-size detrital grains (Fig. 12B). Like that of the Five Mile Lake formation, the Three Lakes formation magnetite is interpreted as derived by in situ metamorphic recrystallization of (an) early diagenetic iron rich mineral(s).
The Three Lakes formation gradationally overlies the Aluminium River formation where the latter is present without the Five Mile Lake formation. It overlies the Resort Lake formation where the Aluminium River formation is absent. Nowhere is it in contact with the Five Mile Lake formation basalt. It is everywhere gradationally overlain by the Oora Lake formation. The lower and upper contacts are gradational over about 30 m. Where it overlies the Aluminium River formation, the interbedded dolomitic portion is included in the Aluminium River formation and strain is less than in either the pure dolomitic marble or pure phyllite. In such places, the lowest unit assigned to the Three Lakes formation is grey laminated phyllitic siltstone and mudstone with local orange- and purple-weathering grey dolostone concretions whose laminae are continuous with those of the enclosing siltstone. Where the Three Lakes formation overlies the Resort Lake formation, the contact is placed where the siltstone becomes a noncarbonaceous grey rather than carbonaceous black, and reaffirmed by the magnetic marker. The lack of magnetic marker in poorly exposed localities suggests that the upper Three Lakes formation is absent.
The upper Three Lakes formation, best exposed at its type transect west of Showing lake, consists of grey green biotite phyllite intercalated with tabular bedded, pink weathering, grey feldspathic quartzarenite. The relatively undeformed sandy beds preserve ripple cross-laminae and trough cross-beds, and the thinner sandy beds are lenticular. This portion of the formation is part of a generally coarsening upward succession interpreted as shallowing upward, and resembles the Showing Lake formation. Given the isoclinal folding of sequence Ps3, it is possible that some localities mapped as Three Lakes formation may be Showing Lake formation. The strong linear aeromagnetic marker that is spatially close to, and parallel with, the better exposed Oora Lake formation is the most reliable guide as to the upper Three Lakes formation. It is distinct from the continuous double moderate aeromagnetic markers of the Showing Lake formation and nonmagnetic Oora Lake formation. Most of the sandstone-hosted uranium occurrences (Fig. 8) are in the Showing Lake formation, a few are in the Oora Lake, but there are none in the Three Lakes formation.
Young (1979) interpreted the Three Lakes formation as deep to shallow subtidal marine deposits, resulting from marine transgression and drowning of the Aluminium River carbonate platform, followed by shallowing upward into the Oora Lake foliated sandstone unit. In Fig. 3, the Three Lakes formation is illustrated diagrammatically as beside and above the Five Mile Lake basalt. However, its magnetic member is interpreted as contemporaneous with the Five Mile Lake basalt. Volcanic exhalative activity is thought to have introduced iron into a deep basin and the early iron phases became magnetite during MP2.
3.3.4.3 Oora Lake formation: foliated arkose and arkosic dolarenite
The Oora Lake formation consists of a lower rhythmic 10 cm scale alternation of grey foliated sandstone and phyllite, succeeded by medium-bedded, pale tan weathering, grey foliated feldspathic quartzarenite, topped by an extensive orange-weathering foliated dolomitic sandstone to quartzose dolarenite marker unit. This foliated sandstone unit is relatively well exposed, shows very little internal strain and contains well-preserved sedimentary structures, even though the enclosing phyllitic Three Lakes and Showing Lake formations are recessive and show intense internal strain. Planar lamination dominates but a wide range of cross-beds is also present. The upper dolomitic marker preserves abundant small cross-beds and possible microbial laminae (X, S, respectively, in Fig. 12C). At 1:30000 and smaller scales the Oora Lake formation defines abundant recumbent isoclines (FP2, P3) (Calhoun et al. 2014). The lower contact is gradational and locally exposed, whereas the upper contact is sharp and rarely exposed. In map width, the formation ranges from 300 to 500 m. The type transect (* in Fig. 8) is continuous with that of the Three Lakes formation. The Oora Lake formation hosts a few sandstone-hosted uranium occurrences (Gandhi et al. 2015).
Young (1979) interpreted the transitional base of the Oora Lake formation above the Three Lakes formation as an intertidal proximal ramp, the middle foliated planar-bedded quartzarenite as a barrier or marginal beach, or a shallow marginal marine environment affected by tides, in contrast to the deeper marine, nontidal (or subtidal) Three Lakes formation. The largest cross-beds may represent aeolian dunes associated with the beach deposits, or large intertidal channels. The interbedded planar fine carbonate beds could have formed in shallow lagoons. Carbonate intraclast breccia and associated large cross-beds may represent storm reworking. Marine transgression with decreased sand supply and continued tidal influence could have developed the dolomitic upper marker unit.
3.3.4.4 Showing Lake formation: interbedded phyllitic siltstone and uranium-bearing foliated calcareous fine arkose
The Showing Lake formation derives its name from the informal “Showing lake” (S in Fig. 8), so named because it is surrounded by multiple stratabound sandstone-hosted uranium occurrences. It is pale grey to salmon pink weathering, grey-green on fresh surfaces, and consists of rhythmically intercalated foliated calcareous arkose and phyllitic argillite, interlayered on a scale of metres to centimetres (Fig. 12D). Much of it is so schistose that compositional layering cannot be discerned. Two linear aeromagnetic highs are weaker than the single high marking the Three Lakes formation, but strong enough to be continuous on unenhanced aeromagnetic data (RTF inset, Fig. 8). All of the known sandstone-hosted disseminated uranium occurrences are coincident with one of the aeromagnetic markers, except a few thought to be within the Oora Lake formation.
The foliated calcareous feldspathic sandstone members of the Showing Lake formation are much less deformed internally than the rhythmically intervening phyllitic argillaceous layers, however they do form recumbent FP1 isoclines (Calhoun et al. (2014). Thick foliated sandstone beds preserve parallel lamination, large to small trough cross-beds and ripple cross-laminae, much like the Oora Lake formation. In contrast, the phyllitic mudstone is highly deformed, with compositional layering transposed subparallel to foliation (SP0 = SP1 in Fig. 12D). Crenulation of SP1 during DP2 involved pressure solution that created pseudocompositional layering (stripes labelled SP2ps in Fig. 12D) which is refracted at the interfaces between sandy and phyllitic layers. The Showing Lake formation is the youngest unit below sequence Ps4 Tahiratuaq group.
The following description of uranium occurrences in the Showing Lake formation is derived mainly from Knox (1980), Blackwell (1978), and Gandhi et al. (2015) in the context of integrated structural geology (Calhoun et al. 2014; White et al. 2021; this volume), regional mapping (Jefferson et al. 2015) and geophysics (Tschirhart et al. 2011b, 2013, 2014, 2017). Uranium was first discovered in the Amer Belt by Aquitaine Company of Canada Limited during ground follow-up of a regional airborne radiometric survey flown in 1968. The occurrences explored by Cominco Corp. (Blackwell 1978) focused on drilling on the Aquitaine properties, whereas Westmin Resources Inc. (Knox 1980) explored farther southwest and initially focused on surface mapping of occurrences. Both sets of sandstone-hosted uranium showings are stratabound, within two continuous members of relatively coarse grained, well-sorted foliated calcareous arkosic sandstone, about 175 m apart, that resemble the Oora Lake formation. The lower foliated sandstone member is 15–40 m above the Oora Lake formation carbonate marker. The two foliated sandstone members are sandwiched by thinly interlayered phyllitic metasiltstone and foliated fine arkosic sandstone that conformably overlie the Oora Lake formation. Uranium mineralization is consistently restricted to these units as far as they are exposed, up to 1350 m along strike. The uranium zones can be followed reliably only by means of scintillometer or spectrometer, although they characteristically have a reddish weathering colour, and the host sandstone is slightly coarser grained than the enclosing strata. From this we infer that the occurrence locations shown in Fig. 8 are more a result of exposure than of actual point locations of mineralization, because extensive heavy drift covers >99% of the Showing Lake formation. Each uranium zone includes one or more distinctly radioactive beds. Grab samples of the most radioactive layers yielded in the order of 0.15% UO2. The disseminated interstitial mineral paragenesis is octahedral magnetite, then pitchblende coeval with pyrite and chalcopyrite, then calcite and quartz cement. Possibly detrital heavy minerals include ilmenite and rutile. Knox (1980) observed that lower uranium content is associated with magnetite, whereas higher uranium is associated with pyrite and chalcopyrite. Blackwell (1978) delineated southwest-plunging cigar shapes of mineralized zones. White et al. (2021) explained these cigar shapes as reflecting concentration by intense fluid flow within DP1 hinge zones, coaxially overprinted by DP2. Post-DP2 hematite alteration post-dates the octahedral magnetite, and includes both specular and earthy phases that give the foliated sandstone its red surface weathering colour. Mineralized samples from both horizons are markedly enriched in U, Pb, Cu, Mo, and S, and weakly enriched in Bi, Co, As, Se, and V relative to the nonmineralized samples.
Additional uranium occurrence types compiled by Gandhi et al. (2015) include fracture-controlled veinlets and a few paleoplacer (monazite + zircon but no detrital uraninite) occurrences in sequence Ps1 (described above). Some uranium occurrences are simply the sites of abundant Dubawnt minette dykes and Martell syenite that are inherently rich in U + Th + REE (Peterson et al. 2011) such as in the R22 area (Fig. 8).
The two uraniferous sandstone members in the Showing Lake formation are coincident with linear aeromagnetic anomalies that provide consistent constraints for both detailed and regional mapping (RTF inset of Fig. 8). Knox (1980) found that the magnetite is euhedral and fills pore spaces, suggesting a diagenetic origin, although the presence of possibly detrital ilmenite suggests a detrital component. Magnetite enrichment in the two sandstone units may have been part of the uranium mineralization process.
Sedimentary structures in foliated calcareous sandstone of the Showing Lake formation suggest a shallow water depositional environment. No primary sedimentary structures are preserved in the phyllitic argillite. Young (1979) suggested either a lagoonal or broad shallow shelf configuration. Similar foliated calcareous sandstone of the underlying Oora Lake formation could have been a consanguineous outer shelf or barrier beach product of marine transgression during abundant clastic input.
3.3.5 Amer sequence Ps4: Tahiratuaq group
Initially described by Knox (1980) as the “upper feldspathic sandstone” and “arkose, coarse grained” units, Young (1979, p. 19) combined these two anchizone–facies units in his “Itza Lake formation” which he described as mainly “pink and grey cross-bedded felspathic (sic) sandstones, purple and red siltstones and claystones, minor orange and buff weathering, locally stromatolitic dolostone and minor boulder and cobble conglomerate”. These are here informally renamed Tahiratuaq group (Fig. 3).
The type area is west of an amoeboid shaped, shallow sandy lake in the southwestern part of NTS 66-G/1 (Figs. 8 and 13) whose Inuit name “Tahiratuaq” means “only a pond” (Pelly and Nales 1986). An extensive washout on the west side of Tahiratuaq provides a nearly continuous reference exposure of the T3 formation for about 3 km obliquely across the southeastern secondary synform. We consider Ps4 in the Amer Belt to be of group status because it has such a wide lithologic range, an apparent stratigraphic thickness of at least 1.2 km (Fig. 6), and unconformably overlies the previously DP1-deformed sequences Ps1 through Ps3, here included in the informal Qikiqtarjualik group (Fig. 3).
Fig. 13.
Fig. 13. Geology of the southwestern Tahiratuaq synform, featuring subdivisions T1 through T4 of the Tahiratuaq group (Ps4). Linear aeromagnetic anomalies in a high-resolution Tilt image (from data reported by Tschirhart et al. 2011a) extend lithologic data from sparse outcrops mapped in 1979 and drill cores logged from 2006 to 2012. Outline shown in southwest portion of Fig. 8. The primary image was exported from a GIS in NAD_1983_UTM_Zone_14 N; WKID: 26914 Authority: EPSG; transverse Mercator projection; coordinates in degrees and minutes.
The Ps4 Tahiratuaq group includes four formational scale units, the first three being mapped at the scale of 1:30000 only in the southwestern part of the belt, by means of three asymmetric aeromagnetic markers that are continuous for about 60 km on an image of the first vertical derivative (Fig. 13). The first three formational units (T1, T2, and T3) occupy the central portion of the broad, upright Tahiratuaq synform that passes under Tahiratuaq and continues southwest beneath the Thelon Formation (Figs. 6, 8, and 13). Other large areas exposing sequence Ps4 lack such magnetic markers and so are undivided. The fourth unit, T4, forms the core of the upright Naujatuuq synform (Figs. 2 and 6) and contains strong linear aeromagnetic markers (Fig. 5). All four formational units preserve pristine sedimentary structures in rhythmically interbedded medium to fine sandstone and mudstone. Young (1979) interpreted the strata in the Tahiratuaq synform and Ps4 graben as shallow marine to subaerial molasse. This is consistent with the Amer sequence Ps4 filling the foreland basin of a rising mountain belt—a product of the early stages of the overlapping DP2.
The basal contact of the Amer Ps4 has not been documented anywhere in outcrop, but is constrained in numerous places by outcrop mapping and geophysical data. It appears to be structurally conformable in a few places but in most places it truncates DP1 thrusts and isoclinal folds. It cuts down in places to at least the Ayagaq Lake formation. At the west end of the Ps4 graben it is exposed within 100 m of Neoarchean granite and greywacke whose schistosity is orthogonal to sandstone bedding. Overall, the mapped geometric relationships document a regional angular unconformity at the base of sequence Ps4.
The Amer Ps4 is unconformably overlain by gently to steeply southeast-dipping Amarook Formation conglomerate and sandstone (Figs. 8 and 13). Both are unconformably overlain by flat lying conglomerate, sandstone and mudstone of the Thelon Formation. South-side-down, dip–slip faults flank the Thelon Formation, as do wedge-shaped paleotalus deposits dominated by blocks of Amarook sandstone. The Tahiratuaq aeromagnetic markers are detectable in places up to 15 km beneath the Thelon Formation.
3.3.5.1 T1 formation: fine siliciclastic rocks
The T1 formation crops out only along the southeastern side of the Tahiratuaq synform in a few glacial–fluvial washout transects. It consists of grey to purple-laminated mudstone overlain by grey mudstone with centimetre-scale graded beds, delicately preserved flame structures, loaded flute casts (Fig. 14A), small ripple marks in siltstone (Fig. 14B), and some chaotically folded intervals. A few discontinuous buff-weathering grey dolostone interbeds punctuate this drab recessive unit. Its aeromagnetic signature is low, and it underlies a linear topographic low along the edge of the Ayagaq Lake formation quartzite. The T1–T2 contact is placed at the first linear aeromagnetic marker, about 200–400 m above Ps1–Ps3 (Figs. 6 and 13).
Fig. 14.
Fig. 14. Lithology of the Ps4 Tahiratuaq group in the southwestern Amer and Naujatuuq belts. Locations of (A) through (F) are labelled in Fig. 13; the remainder are from locations shown in Fig. 8. (A) Flute casts preserved on the base of a siltstone bed, near the base of T1 formation, southeast side of Tahiratuaq synform, southwest Amer Belt (NRCan Photo DB Number 2022-422G.M. Young in 1979). (B) Float of flat-topped ripples near base of T1 formation, same locality as A (NRCan Photo DB Number 2022-423, by G.M. Young in 1979). (C) Typical brick red mudstone interbeds and intraclasts in pink lithic feldspathic sandstone of the T2 formation on the northwest side of Tahiratuaq synform (station 10JPC002) (NRCan Photo DB Number 2022-424, by C.W. Jefferson in 2010). (D) Compacted sand-filled shrinkage cracks or clastic dykes (T2 formation, east-central NTS 66G2) (NRCan Photo DB Number 2022-425, by C.W. Jefferson in 1979). (E) Rectilinear sand-filled cracks (dark red) in pale red siltstone (T2 formation, east central NTS 66G2) (NRCan Photo DB Number 2022-426, by C.W. Jefferson in 1979). (F) Upper part of eastern outcrop of pristine polymict conglomerate grading up to salmon red sandstone at the base of T3, northwest side of axial peaked anticline. Ay = Ayagaq quartzite, fs = feldspathic sandstone intraclasts, FP1 = previously folded (NRCan Photo DB Number 2020-880, by G.M. Young in 1979). (G) Rhythmically alternating pink sandstone and maroon mudstone of undivided Tahiratuaq group at the east end of the Ps4 graben (Fig. 8), NTS 66H5-H6 (NRCan Photo DB Number 2022-427 by G.M. Young in 1979). (H) Low-angle cross-laminae in outcrop of T4 pink-grey fine sandstone, east-central Naujatuuq Belt (NRCan Photo DB Number 2022-428 by C.W. Jefferson in 2008). Photos (I, J, K) by C.W. Jefferson in 2008, of drill core RHC-07-01 (Figs. 2 and 8) shared by Western Uranium Corporation. (I) Graded siltstone–mudstone couplets at 478.5′. Dotted yellow line outlines a disharmonic fold (DP2) (NRCan Photo DB Number 2022-429). (J) Coarse pink sand matrix to intraclasts of green argillite at 210′ (NRCan Photo DB Number 2022-430). (K) Dark grey feldspathic–lithic siltstone and carbonaceous slate at 850′ interpreted as a weak conductor (NRCan Photo DB Number 2022-431).
3.3.5.2 T2 formation: rhythmic red beds
The second formation of the Tahiratuaq group consists of poorly exposed, interbedded lithic feldspathic sandstone, and maroon mudstone, with some orthoquartzite interbeds. Common sedimentary structures include red mudstone intraclasts (concentrated in the lower parts of sandstone beds), ripple marks, small trough cross-beds, desiccation cracks, and small clastic dykes (Figs. 14C–14E). The base of T2 is placed at the first linear aeromagnetic marker (T1/T2). The top of this unit is placed at the base of a relatively strong linear aeromagnetic marker that can be seen faintly in places on RTF images, but is very clear and asymmetric on tilt angle images (T2/T3). It has a map width of about 500 m to 5 km and is approximately 500 m thick (Fig. 6). It crops out rarely along the southeast and northwest sides of the Tahiratuaq synform, and sparsely along the “Axial fault + peaked antiform”.
3.3.5.3 T2–T3 transition: conglomerate and carbonate members
Young (1979) mapped undeformed polymict orthoconglomerate (Fig. 14F) at three sites that coincide with the T2/T3 linear aeromagnetic marker: “Reference outcrops, T2–T3 conglomerate” and “Conglomerate felsenmere” in Fig. 13. The upper contact of the well-preserved conglomerate grades into feldspathic sandstone at the reference outcrops. Rounded pebbles to boulders up to 30 cm in diameter comprise previously foliated and folded white and pink bedded and massive quartzite identical to rock types in the underlying Ayagaq Lake formation, and minor pink porphyritic rhyolite interpreted as 2.6 Ga Pukiq Lake formation. Subangular intraclasts (mainly feldspathic sandstone, with minor pink orthoquartzite, orange-weathering dolostone, and green and buff mudstone) represent the enclosing Ps4 strata. The conglomerate represents syn-Ps4 uplift and regional to local transport of the Amer supergroup and basement.
A thin unit of interbedded desiccation cracked red mudstone and orange-brown weathering, deep brown desiccation cracked dololutite, and locally stromatolitic dolostone, gradationally overlies rhythmically bedded pink sandstone and red mudstone of T2 in several places. In places that lack dolostone, some of the upper T2 beds are calcareous lithic feldspathic sandstone to arkose. Like the conglomerate, the calcareous and dolomitic beds are spatially associated with the T2/T3 linear aeromagnetic marker. The relative stratigraphic positions of the conglomerate and carbonate members require further investigation.
3.3.5.4 T3 formation, rhythmic red beds
The T3 formation is the uppermost unit in the Amer grand synform, where they are about 1 km thick, and form the cores of the two secondary synforms of the broad Tahiratuaq synform. The base of the T3 formation is placed at the base of the strongest and most clearly asymmetric linear aeromagnetic marker (T2/T3) with its spatially associated conglomerate and carbonate members. Much of the T3 formation has a similar lithologic composition to that of the T2 formation: interbedded pink feldspathic sandstone and orthoquartzite with unimodal cross-beds and ripple marks, and interbeds and intraclasts of deep red mudstone. T3 is distinguished from T2 in that it overlies the T2/T3 conglomerate which is interpreted as representing a sequence boundary. The two formations are mapped separately (Figs. 6 and 13) based on their superposition and distinct continuous linear aeromagnetic markers. The upper half of T3 (T3u) is delineated by the uppermost aeromagnetic marker. It appears to be more feldspathic, corresponding to the “arkose unit” of Knox (1980) who estimated its thickness as 500 m. The T3u member also includes a single occurrence of “Upper T3 conglomerate” that has not been described except as a notation on Young’s (1979) outcrop map.
More than 400 cross-beds in the T2 and lower T3 formations record a wide scatter of paleocurrents with modes in the northwest and west–southwest (Young 1979), suggesting a tidal influence. The scattered to abundant intraclasts of red, green, and purple mudstone incorporated in the lower parts of sandstone beds (Fig. 14C) could have formed by desiccation and reworking of red mud in a fluctuating fluvial to shallow subtidal environment. The desiccation cracked and locally stromatolitic dolostone also suggests an oxidized subtidal environment. The local orthoconglomerate members are interpreted as high-energy fluvial thalwegs.
Thirty-four cross-beds measured by Young (1979) in upper T3 formation red sandstone on the west side of Tahiratuaq (T3u, Fig. 13) gave consistent northeasterly transport direction. In the same area, other Westmin geologists in 1979 measured 25 ripple crests showing a preferred east–northeast direction and 32 trough cross-beds recording northwesterly transport at this same type transect, consistent with observations made by the first author in 2011. These paleocurrents suggest dominantly alluvial transport involving broad shallow streams.
3.3.5.5 T4 formation
Outcrop in the northwestern part of the R22 synform (Figs. 2 and 9) continues westward into the Naujatuuq Belt, the southeastern part of which exposes 25–100 cm thick beds of pinkish grey-green, fine sandstone (Fig. 14H) that are rhythmically intercalated with 5 cm beds of laminated grey-green mudstone (Fig. 14I). Moderate dips in this area define open southwest-plunging FP2 folds. Three set of core stored at a former camp site of Western Uranium Corp. on the west side of Sand Lake (Fig. 2) serve as lithostratigraphic references in this area: RHC-07-01, RHC-07-02, and RHC-07-03 (labelled RHC-07-01 in Figs. 5, 6, and 8). The drill holes intersected up to 1225 ft. (373 m) of moderately dipping beds with weakly deformed primary sedimentary structures and textures. Drill core indicates thinner bedding at depth, where the alternating grey-green siltstone–mudstone beds are in the order of 1 cm and the pale pink to green sandstone beds range from 5 to 30 cm. Low-angle to festoon cross-beds (Fig. 14H) are present in outcrops of pinkish green medium sandstone. Centimetre-scale pink sandstone dykes, like those in the T3 formation, are common, as are climbing ripples at the tops of graded sandstone beds. The clastic dykes range up to 3 cm across (Fig. 14J) and penetrate as much as 15 cm of strata. Where they transect thick mudstone beds, the clastic dykes are folded, with axes oblique to bedding and containing axial planar SP2 sericitic foliation. Deformation is also expressed as shearing and brecciation of multiple 1– 5 m intervals of graphitic–pyritic black slate (Fig. 14K). The black slate layers are interpreted as the weak conductors targeted by drilling. The area to the west of the RHC-07-01 site has multiple strong linear aeromagnetic anomalies of unknown origin.
The pinkish grey-green rhythmites and their linear aeromagnetic markers are interpreted as moderate to deep-water turbidites, seismites, contourites, and/or tempestites. The pinkish sandstone component and the angular intraclasts of red and purple mudstone may have been generated in an oxygenated shallow intertidal to subaerial environment before transport into deeper water less oxidizing conditions. Green intraclasts of similar shape, size, and composition mixed with the red and purple intraclasts may have been reworked from subaqueous green beds. All or part of the T4 formation may overlie or be laterally equivalent to the T1 to T3 formations.
3.3.5.6 Areas of undivided sequence Ps4 within the Amer Belt
A number of small to very large outliers of sequence Ps4 elsewhere in the Amer Belt each have slightly different lithofacies, and lack known aeromagnetic anomalies. Although all have lithic feldspathic compositions, similar detrital zircon geochronology, angular discordance with underlying Ps1 through Ps3 strata, and a lack of DP1 folds and fabrics. In the northeast oval (northeast of station 10JPC165) a small oval outlier of sequence Ps4 contains weakly foliated, moderately dipping, brown-graded sandstone, and laminated mudstone couplets interpreted as turbidites. A sample from this locality yielded detrital zircon as young as 2.04 Ga (Davis 2021). A decametre sized outlier west of upper Amer Lake exposes subhorizontal, ripple-marked feldspathic sandstone unconformably straddling the contact between vertical sequence Ps1 orthoquartzite and Ps2 Resort Lake formation within the Fifty Mile syncline. South of Showing lake, weakly foliated salmon pink feldspathic sandstone yielded detrital zircon as young as 1.91 Ga (Rainbird et al. 2010, z8985). Northeast of Five Mile lake, weakly deformed polymict orthoconglomerate unconformably overlies isoclinally folded sequence Ps1 quartzite. The shape of the conglomerate resembles an upright, open FP2 syncline that plunges gently to the southwest and terminates abruptly to the west against a 160° fault.
The Ps4 graben cuts across DP1 isoclinal folds, basement-involved imbricate thrusts of sequences Ps1–Ps3, and fabrics in the Archean rocks at its northwest end. Pink sandstone from the east end of this graben (Fig. 14G illustrates bedding in this area) yielded the youngest known sequence Ps4 detrital zircon (1.9 Ga, Rainbird et al. 2010).
The Tahiratuaq group is also undivided within the broad R22 synform. Miller (1995) recognized these rocks as “upper Amer Group” and compared them to the main fill of the doubly plunging Garry Lake synform. Tella (1994) mapped them as “PAfs: feldspathic sandstone, calcareous arkose, interbedded arkose–siltstone–slate–mudstone”. Exploration in this area found uranium occurrences and a conductor at Ifo lake (Fig. 8) that could be in the Resort Lake formation or Ps4. Tschirhart et al. (2014, 2017) modeled up to 250 m of undifferentiated Ps2 to Ps4 at the northwest end of a gravity transect (Fig. 6) that ends within the R22 synform.
3.4.1 The Late Paleoproterozoic Dubawnt Supergroup and associated igneous rocks
Upper portions of the Dubawnt Supergroup (the Amarook and Thelon formations) cover the middle part of the Amer Belt in the Aberdeen Subbasin (Fig. 2) (Jefferson et al. in press). The Dubawnt Supergroup (upper part of Fig. 3) comprises the Baker Lake, Wharton, and Barrensland groups; all of which preserve pristine primary sedimentary and volcanic textures and structures, and are undeformed except adjacent to fault zones. Rainbird et al. (2003) interpreted these groups in terms of three major depositional sequences. Peterson et al. (2010) and Scott et al. (2015) noted that the volcanic strata intercalated within the first two groups are products of two large igneous provinces—the 1.84–1.81 Ga Hudson suite and 1.77–1.73 Ga Kivalliq igneous suite, respectively.
The Baker Lake Group fills the Baker Lake Basin (inset, Fig. 1) and a series of subbasins extending to the southwest (not shown here) (Rainbird et al. 2003). Pristine red fluvial conglomerate, arkose and mudstone of the Kazan and South Channel/Angikuni formations are intercalated with mafic to felsic ultrapotassic volcanic rocks of the Christopher Island Formation and unconformably overlain by conglomeratic alluvial red beds of the Kunwak Formation (Aspler et al. 1999; Rainbird et al. 2006). Rainbird et al. (2006) and Rainbird and Davis (2007) bracketed Baker Lake Group deposition between a 1833.2 ± 2.4 Ma lower felsic minette flow of the Christopher Island Formation and a 1783 ± 3 Ma calcite travertine bed near the top of the Kunwak Formation. The first-order Baker sequence was deposited contemporaneously with the 1.84–1.81 Ga Hudson suite granite and Martell syenite that are mingled with, and cut by, multiple swarms of mafic to felsic minette dykes that are petrochemically equivalent to the Christopher Island Formation (Scott et al. 2015).
The Wharton Group unconformably overlaps the Baker Lake Group within the western Baker Lake Basin, forms flat-lying cover to fault-tilted slivers as far as the eastern margin of the Thelon Basin, and is made up of the Amarook and overlying Pitz formations (Rainbird and Hadlari 2000). The Amarook Formation comprises undeformed thin polymict basal conglomerate overlain by alternating feldspathic quartzarenite and polymict conglomerate. The sandstone includes alluvial and aeolian facies, both being cemented by clear and cryptocrystalline quartz. The Pitz Formation abruptly overlies the Amarook Formation with interpreted disconformity and is the extrusive portion of the second large igneous province of the Dubawnt Supergroup—the Kivalliq igneous suite. The Pitz Formation is restricted to the south side of the Aberdeen Subbasin and its upper member flanks the east side of the main Thelon Basin where it comprises undeformed intercalated conglomerate and epiclastic sandstone. Clasts of the conglomerate include about 30% volcanic rocks of the lower Pitz Formation, 60% sedimentary rocks including the Amarook Formation, and 10% Archean granitoid and volcanic rocks (LeCheminant et al. 1983; Rainbird and Hadlari 2000; Jefferson et al. 2013, in press).
The Barrensland Group comprises clay-altered red alluvial lithic–arkosic siliciclastic strata of the Thelon Formation (Fig. 3), a small patch of thin 1540 ± 30 Ma ultrapotassic mafic flows and tuff of the Kuungmi Formation (Gall et al. 1992; Peterson 1995; Chamberlain et al. 2010) located at the intersection of the Bathurst and McDonald faults (Fig. 1; not shown in Fig. 3), and patches of silicified dolostone of the Lookout Point Formation (Gall et al. 1992) (not broken out in Fig. 1 but located near the middle of the Thelon Basin). Cecile (1973), Hiatt et al. (2003), Palmer et al. (2004), Davis et al. (2011), Jefferson et al. (2011a, 2015, in press) provided descriptions and interpretations of the Thelon Formation, including references to previous work and evidence for reactivated faulting before, during, and after three depositional cycles, clay alteration, and mineralization events. Clasts in basal conglomerate of the Thelon Formation tend to reflect the lithology of the local underlying rocks, for example, blocks of silicified aeolian sandstone derived from the Amarook Formation or basement gneiss at different localities around the margins of the Aberdeen Subbasin (inset to Fig. 1).
Paleocurrent data from the Thelon Formation (Donaldson 1965; Cecile 1973; and this project) are overall unimodal westerly to southwesterly, with subordinate orthogonal directions in finer grained facies. Together these data support the “big river” concept of Rainbird et al. (2003a) in terms of a major alluvial system that trended west–northwest from Baker Lake to fill the Aberdeen Subbasin, then south to fill the main part of the Thelon Basin and west again toward the East Arm of Great Slave Lake. Tributaries flowed into this main channel from the south, east, and west. The western side of the Thelon Basin was bounded by residual uplands over the area of the Thelon tectonic zone.
4 Age controls and provenance of sequences Ps1 through Ps4
Many of the age constraints in the Amer Belt are relative (cross-cutting elements, structure, and superposition). Reliable detrital zircon geochronology for the Amer Belt (summarized in Fig. 3), is limited to Ps1 and Ps4. A new Re–Os age for Ps2 is reported here. Additional geochronology for the Montresor and Ketyet River belts is compared here, assuming that the four sequences of the Amer Belt are correlative with those of the Montresor and Ketyet River belts. Data on ages, samples, and methods are summarized in Jefferson et al. (in press); in particular, their table 1 comprehensively integrates all known depositional, magmatic, structural, and metamorphic events in the region.
Evidence for local provenance includes the presence of clasts in conglomerate that are lithologically the same as units in nearby basement rocks, with relative proximality indicated by grain size. Changes in sea level cannot be distinguished from tectonic uplift or subsidence, except that the presence of alluvial conglomerate (orthoconglomerate associated with trough cross-beds) is here inferred to reflect the proximality of syndepositional fault-generated paleotopography. Distal provenance is inferred in siliciclastic strata mainly from detrital zircon ages that are not represented by dated rocks in the study area.
4.1 Sequence Ps1
Detrital zircon in the Amer Ps1 range from 3.7 to 2.6 Ga. No 2.3 Ga zircon are present, even though the lack of Arrowsmith (Berman et al. 2013) deformation and metamorphism throughout the study region (Jefferson et al. in press) indicate that sequence Ps1 post-dates that orogeny. The strong youngest peak of 33 detrital zircon grains centred on 2.19 Ga in the Montresor Ps1 (Percival et al. 2017) is consistent with that interpretation. The Neoarchean-dominated detrital zircon reflect local sources (Rainbird et al. 2010). The 2.19 Ga detrital zircon peak in the Montresor Ps1 may have been derived locally from an unknown bimodal magmatic suite (Percival et al. 2017) and has not been detected in the Amer or Ketyet River Ps1, but has analogues in a number of other Rae Craton cover sequences (Davis et al. 2021).
The extensive stratabound extent of the red transition between the basal conglomerate and overlying vitreous quartzite is consistent with the <2.3 Ga age of Ps1—after the great oxyatmoversion event constrained by the Huronian Supergroup to between 2.45 and 2.2 Ga (e.g., Prasad and Roscoe 1996; Shawwa et al. 2021). The presence of red argillite with desiccation cracks interbedded with red-matrixed conglomerate and pink feldspathic quartzite in the least-metamorphosed southwestern parts of the Amer Belt are the most obvious record of an oxidizing atmosphere during deposition, however, the pink feldspathic quartzite unit is also present in the area northeast of lower Amer Lake which is at upper greenschist metamorphic grade. The red colour in these occurrences is related to finely disseminated hematite throughout the rock. The presence of this stratabound unit far from any possible effects of the unconformity at the base of the Thelon Formation, and the lack of reddening of the paleoweathered granite in the same areas argue against the reddening being a post-metamorphic effect. The lack of observed red beds at the Amer Lake type transect may be due to primary depositional/diagenetic differences and/or higher metamorphism.
4.2 Sequence Ps2
A new Re–Os age of 2126 ± 24 Ma (Fig. 15 and Appendix A) was obtained for drill core samples of black slate from the Resort Lake formation, located at the western edge of the Naujatuuq Belt (DPR-10, Fig. 2) beneath about 150–200 m of Thelon Formation (profile L11 in fig. 5 in Tschirhart et al. 2014). The graphitic–pyritic slate contains up to 15% TOC (TOT/C, Appendix B). Samples taken for geochronology (98-RAT-1, 2, 4, 5, and 6) represent the source of an electromagnetic conductor targeted by exploration drilling by Cameco Corporation for unconformity-related uranium mineralization. The final Re–Os result is based on sample 98-RAT-1 which has the highest carbon content.
Fig. 15.
Fig. 15. Re–Os isochron age of the Resort Lake formation (sequence Ps2). Samples from drill hole DPR-10 in the subsurface western edge of the Naujatuuq Belt (Fig. 2). Data in Appendix A.
The Re–Os data for the age determination were obtained in 2009. The analytical uncertainty on this age is attributable to the high mean square-weighted deviation (Fig. 15), meaning that it is possible that the initial Os was changing throughout the deposition period sampled with the core. One analysis (98RAT-1(4)), which plots well below the isochron, is omitted from the preferred regression age of 2126 ± 24 Ma. The initial Os ratio is nominally subchondritic, but extends to values of 0.12–0.13 including the uncertainty estimate, suggesting Os input into the water column was dominantly mantle/hydrothermal, which at 2120 Ma would have had Os of ca. 0.115.
This new age is consistent with interpretation of the unit as in the lower sequence Ps2 Resort Lake formation, based on its lithology and geophysically inferred position adjacent to Ps1 quartzite along the western edge of the buried southwest extension of the Naujatuuq Belt (Fig. 2). It predates the 2.05 Ga Shunga euxinic carbon depositional event recorded in the upper Zaonega Formation of the Onega Basin in Karelia (Asael et al. 2013), the Cod Island Formation of the Mugford Group in Labrador (Wilton et al. 1993; Wilton 1996; Craig et al. 2013) and the lower-mid Union Island Group (Hoffman et al. 1977) that is overlain by 2045.8 ± 1.0 Ma volcanic rocks (Sheen et al. 2019). This supports the hypothesis of multiple Paleoproterozoic carbon burial episodes (Martin et al. 2015).
Percival et al. (2017) bracketed the age of a structural sliver of micaceous schist and wacke in the northeast Montresor footwall complex between a boudinaged 2045 ± 13 Ma metagabbro sill within it, and the younger than 2.19 Ga sequence Ps1 quartzite below it. They interpreted the micaceous schist and wacke as sequence Ps2. Percival et al. (2019) reported a ca. 2.1 Ga (U–Pb zircon) impact spherule bed within the same structural sliver. The lack of an aeromagnetic marker in the Montresor schist and wacke, the gabbro and spherule ages and its spatial juxtaposition with the Ps1 quartzite further support its interpretation as lower Ps2, contrary to the Ps3 interpretation of Jefferson et al. (in press).
4.3 Sequence Ps3
There are no reliable direct age data for sequence Ps3 units in the Amer and Ketyet River belts, and no Ps3 is documented for any of the other Paleoproterozoic belts in the study area (Fig. 2). Its maximum age is that of Ps2 −2126 ± 24 Ma. Based on the structural paradigm (Fig. 4), the DP1 structures in Ps3 constrain its deposition to before the 1.9–1.865 Ga Snowbird orogeny (Pehrsson et al. 2013b).
One age previously considered for sequence Ps3 is 2153 ± 5 Ma (A.N. LeCheminant and J.C. Roddick, unpublished data, cited in Hadlari et al. 2004) on the “Schultz Lake metagabbro” slab that crosscuts Neoarchean metagreywacke southeast of the Schultz Lake klippe (Figs. 2 and 16) (Jefferson et al. in press). However, the basalt differs geochemically from the gabbro (Patterson et al. 2012) and 2153 ± 5 Ma is too old with respect to our new 2126 ± 24 Ma Re–Os age on the Ps2 Resort Lake formation. The 2153 ± 5 Ma age is more consistent with the inferred <2.15 Ga age of pillow basalt in the Rankin Inlet greenstone belt that conformably overlies and is depositionally intercalated with (S. Pehrsson (personal communication, 2022)) schistose paraconglomerate and greywacke that have a maximum depositional age of 2155 ± 5 Ma (Davis et al. 2008). The Paleoproterozoic volcanic and sedimentary strata in the Rankin Inlet belt are considered to be part of the southeast margin of the Chesterfield block and thus part of the Rae Craton (Pehrsson et al. 2013b).
Fig. 16.
Fig. 16. Generalized correlation, comparisons, and contrasts of Paleoproterozoic strata in the central Rae Craton; not to scale. See Fig. 3 and text for unit descriptions and discussion.
Earthy brown weathering mafic minette dykes, both scattered and in a swarm transecting the R22 area (Peterson et al. 2011) and the Tahiratuaq synform, post-date FP2 folds and thrusts and the Ps4 Tahiratuaq group (Fig. 8). They weather recessively, resembling paleosol, and source trains of glacially transported blocks. They are texturally and geochemically identical to Dubawnt minette dykes (Peterson and LeCheminant 1996) that fed 1833 ± 3 Ma to 1811 ± 12 Ma Christopher Island Formation mafic and felsic minette flows (Rainbird et al. 2003, 2006) interlayered with the Kazan arkose of the Baker Lake Group, however minette flows are not present in Amer sequence Ps4. Dykes of the R22 swarm range from 10 to 150 cm in width and contain several hundred ppm Th + U (Peterson et al. 2011), yet there is neither hydrothermal alteration nor uranium mineralization in adjacent strata. In contrast, the widespread minette dykes cutting the Kazan Formation range from 3 to 10 m across and have spatially associated hydrothermal sodic alteration and uranium mineralization (Stanton 1979; Miller 1980). These differences help distinguish Amer sequence Ps4 from similarly well-preserved red beds of the Baker Lake Group.
A key difference of the Thluicho Lake and Nonacho belts is their lack of sequences Ps1 through Ps3. Conversely, the Murmac Bay group (WB in Fig. 1, described by Bethune et al. 2013) does have units and deformation similar to the Amer Ps1–Ps3 but not Ps4. The 2.06–2.0 Ga detrital zircon of Ps4 likely reflect the widespread magmatism of that age noted above for Ps2. The sparse 1.92–1.90 Ga detrital zircon of Ps4 likely represent the ca 1.9–1.865 Ga DP1 Snowbird event (Rainbird et al. 2010). A 1.92–1.90 Ga detrital zircon in the 1833 ± 3 Ma to 1785 ± 3 Ma Baker Lake Group may have been recycled from Ps4 (Rainbird et al. 2007) or derived directly from the Snowbird orogen.
5 Lithostratigraphic comparisons of Paleoproterozoic strata, Rae Craton
The following lithostratigraphic and metallogenic comparisons are mainly between the Paleoproterozoic belts in the central Rae Craton (Fig. 16), with some consideration of other parts of the Rae. In the central Rae Craton, both the youngest basement rocks, and the youngest deformation and metamorphism beneath the Paleoproterozoic belts are Neoarchean. In contrast, the north and south Rae Craton are strongly affected by 2.3 Ga metamorphism, for example, Paleoproterozoic quartzite in the Committee Bay Belt (Berman et al. 2010; Sanborn-Barrie et al. 2014) and the Murmac Bay Group (Bethune et al. 2013). Supracrustal rocks of the Neoarchean Woodburn Lake group underlie only the Ketyet River and Amer belts, although the footwall complex of undated muscovite–andalusite–sillimanite–garnet schist that underlies the southeast side of the Montresor Belt (Frisch 2000; Dziawa et al. 2019) may be age equivalent. Augen gneiss and foliated granite of the SIs underlie all of the Paleoproterozoic belts compared here, except for the west side of the Garry Lake Belt that is underlain by the undated Garry Lake complex of partially melted supracrustal rocks intercalated with granitic leucosomes. The Pukiq Lake formation, extrusive phase of the SIs, underlies marginal parts of only the Amer, Ketyet River, and Akiliniq Hills belts. Garnet muscovite granite with inclusions of biotite schist and amphibolite, all of suspected Archean age, underlie sequence Ps4 at the southwest end of the Montresor Belt (Tella 1994; Frisch 2000).
Preservation of the Pukiq Lake formation, the four Paleoproterozoic sequences and the Dubawnt Supergroup (Fig. 1) was favoured in the central Rae Craton. Together with the absence of the 2.3 Ga Arrowsmith orogeny, this suggests that the central Rae Craton has undefined fundamental crustal differences from the north and south Rae Cratons. In any case, the preservation of cover rocks and their associated mineral occurrences in the central Rae Craton, like that of Athabasca Basin region, is what makes these areas so economically important.
Numerous exposures of the basal sequence Ps1 schistose conglomerate compositionally reflect the local Neoarchean volcanic and intrusive rocks in the Amer and Ketyet River belts (Pehrsson et al. 2013b; Davis 2021; Jefferson et al. in press). In the Montresor Belt, the Ps1 schistose conglomerate is known only near the southwest corner of the grand synform, where it is highly strained, contains traces of native copper (Cu in Fig. 2) and overlies massive garnet–muscovite granite. Basal conglomerate is not documented in the other belts of the study area. The profound unconformity at the base of Ps1 is a locus of high strain along much of its length, including thrusting, albeit not as intense as in the Murmac Bay Group as described by Bethune et al. (2013).
The highly deformed Ps1 quartzite is continuously exposed in all of the Ketyet River Belt and the Akiliniq Hills, but is discontinuous to absent beneath Ps4 in the Montresor, Naujatuuq, Sand, and Garry Lake belts (Tella 1994; Miller 1995; Frisch 2000; Percival et al. 2017). The quartzite and Ps2 marble form DP1 isoclinal folds and imbricate thrusts in the northeastern Montresor Belt, like their counterparts in the Amer and Ketyet River belts (interpretation by the first author of maps by Frisch 2000 and Percival et al. 2017). The Ps1 quartzite is absent in the Amer Belt only at the west end of the Ps4 graben. Discontinuities of the quartzite are interpreted as due to erosion before deposition of sequence Ps4. Similar aged quartzite is present across the Rae Craton (Rainbird et al. 2010).
Lateral variations of the Ps1–Ps2 transition in the Ketyet River Belt are more pronounced than those described above for the Amer Belt. For example foliated 20–30 cm thick graded beds with ball and pillow structures, and schistose conglomerate pods are intercalated with the lower Ps2 rusty weathering phyllite in at least two places (Jefferson et al. in press). The foliated iron formation mineralogy includes silicate, carbonate, sulfide and nonmagnetic hematite. Graphitic conductors are also present. Hematite-chert beds separated by 2–3 cm phyllitic partings are present in Ps2 along the east side of the Schultz Lake klippe (McEwan 2012) and in the Akiliniq Hills (LeCheminant et al. 1984). Zaleski and Pehrsson (2005) mapped this transition as APWif and reported a single occurrence of galena with trace chalcopyrite in massive pyrite–pyrrhotite–galena east of Whitehills Lake, as well as a number of locally anomalous concentrations of one or more of As, Bi, Mo, Sb, and W. The above-dated samples of Resort Lake formation are also slightly metalliferous, with locally elevated Ag, As, Au, Bi, Cu, Mo, Ni, Pb, and V (Appendix B).
The sequence Ps2 highly strained marble of the Montresor Belt is remarkably similar to that of the Amer Belt, as described by Frisch (2000) and Patterson (personal communication, July 2009). The recessive zone between quartzite and marble is thin and barely mentioned in reports by Frisch (2000) and Percival et al. (2017), but may represent graphitic phyllite like the Resort Lake formation. The marble wraps around the northeast end of the Montresor synform and is present in the imbricate zone northeast of the closure, but is absent beneath Ps4 in the southwestern two thirds of the Montresor synform and outliers. In the Ketyet River Belt, the marble is mostly absent except for a thin calcareous mafic schist in the Schultz Lake klippe (McEwan 2012; Fig. 2), dolomitic limestone in the core of a quartzite synform east of Quoich River (Fig. 1D; Fraser 1988), and small outliers in gneiss between the Whitehills thrust and Chesterfield fault zone (faults shown on inset map of Fig. 1). No Ps2 marble is known in the other central Rae belts; in the Murmac Bay Group the marble unit overlies mafic volcanic rocks (Ashton et al. 2013; Bethune et al. 2013) rather than underlying the basalt as in the Amer Belt.
The sequence Ps3 Ketyet River group foliated basalt is stratigraphically and texturally indistinguishable from the Five Mile Lake formation of the Amer Belt, both being calcareous, amygdaloidal, with swallowtail plagioclase phenocrysts and having unimodal tholeiitic compositions. Both are the youngest units in the cores of FP1 isoclinal synclines, have lower tuff and upper massive flows, and are laterally discontinuous along northeast trends. The Ketyet River foliated basalt in the Whitehills FP2 synform forms the cores of extremely tight isoclines within quartzite, such that the basalt cores resemble interbeds. Nevertheless, each basalt core was verified as an FP1 syncline by flattened upper Ps1 conglomerate and thin layer of Ps2 dark phyllite separating the white quartzite from both sides of the foliated basalt (Jefferson et al. in press). The minor and trace elements of the Ketyet River foliated basalt differ slightly from those of the Amer foliated basalt, and both basalt units are geochemically distinct from altered recrystallized gabbro sills in the two belts (Patterson et al. 2012). No foliated basalt is reported from the other belts in the study area. Foliated mafic volcanic rocks in the Murmac Bay Group directly overlie <2.33 Ga quartzite and are overlain by highly strained marble, then <2.17 Ga psammopelitic gneiss (Ashton et al. 2013; Bethune et al. 2013).
The undated upper sequence Ps3 (upper Three Lakes, Oora Lake, and Showing Lake formations) is restricted to the Amer Belt. The other central Rae belts lack intense linear aeromagnetic anomalies in fine siliciclastic rocks like the Three Lakes formation, and there are no reports of stratabound sandstone-hosted uranium occurrences in foliated calcareous arkosic sandstone and phyllite like those in the Oora Lake and Showing Lake formations, even though all of these belts have been explored by expert prospectors. Although a magnetic marker has not been described for the upper <2.17 Ga pelitic–psammopelitic gneiss of the south Rae Murmac Bay Group (Ashton et al. 2013; Bethune et al. 2013), it may be age equivalent to the Three Lakes formation, although deposited a cratonic margin rather than the interior setting for Ps3, and being affected by the Taltson orogen. Possibly also contemporaneous with upper Ps3, the 1.95–1.91 Ga Assemblage II of the western Rae Ellice River belt was a more proximal, rift basin recipient of detritus from the uplifted and exhumed Thelon arc (Davis et al. 2021).
Sequence Ps4 forms about two-thirds of the map width of the Amer Belt, about half of the Ketyet River Belt in the Whitehills synform, and is the dominant unit of the remaining belts where in many places it directly overlies Archean rocks. The basal contact is exposed on the west side of the Whitehills synform of the Ketyet River Belt, where dark grey slate mantles the irregular eroded surface of isoclinally folded Ps1 quartzite with a sharp contact and no conglomerate. Elsewhere, the geometry of the basal Ps4 contact is constrained between sparse outcrops, by the structural paradigm and by high-resolution geophysical data. None of the drill holes logged in the GEM project intersected the unconformity. In the absence of quartzite, the Garry Lake Belt is best outlined by a continuous conductor in a lower graphitic slate that Miller (1995) assigned to the lower sections of the upper Amer group (sequence Ps4) based on its similarity to the upper Amer group in areas mapped by Tella (1994).
The description by Frisch (2000, p. 26) for the Montresor Belt is apt for all of Ps4 in the study area: “Nowhere was deformation intense and nowhere is basement intercalated with Montresor rocks”. Preservation of primary sedimentary structures is excellent. The Amer and Garry Lake belts are dominated by subaerial to shallow marine red feldspathic molasse (see detailed petrographic descriptions by Knox 1980 and Miller 1995, respectively). The other belts of Ps4 comprise pink-grey-green to grey shallow subaqueous feldspathic flysch. The Amer, Ketyet River, and Montresor belts expose one or more intrasequence Ps4 pristine orthoconglomerate members that include well rounded clasts of DP1—deformed quartzite (Fig. 14F; Jefferson et al. in press).
Magnetic markers of the Naujatuuq and Amer belts Ps4 are described above. There are no Ps4 magnetic markers west of the Naujatuuq Belt (Fig. 5). The Deep Rose Belt has one strong but discontinuous marker. The Montresor sequence Ps4, from bottom to top, has two strong continuous, one weak continuous, several weak discontinuous, and two very strong continuous aeromagnetic markers (Tschirhart et al. 2015; Pilkington and Tschirhart 2017). Discontinuities in the markers are interpreted as faults (Tschirhart et al. 2015; Percival et al. 2017; Jefferson et al. in press). The uppermost Montresor marker corresponds to a paleoplacer heavy mineral layer comprising silt-sized detrital magnetite, apatite, tourmaline, and zircon (Percival et al. 2017). In the Ketyet River belt, the upper sequence Ps4 has a lower discontinuous strong marker and an upper continuous strong marker over a strike length of 20 km, with some weaker discontinuous strands. All of the sequence Ps4 magnetic markers are inferred to be detrital magnetite like that of the upper Montresor group, genetically distinct from the euhedral disseminated magnetite of the Three Lakes formation with no heavy minerals (Fig. 12B). In the Showing Lake formation, the euhedral magnetite appears to have grown interstitially between the sand grains, and to have been replaced by pitchblende, pyrite, and chalcopyrite (Knox 1980). Detailed study of all sequence Ps4 magnetic markers is recommended.
Because sequence Ps4 progressively overlies and truncates underlying sequences Ps1 through Ps3 strata toward the western part of the central Rae Craton, and because the mapped contact is folded across multiple DP2 antiforms and synforms, it is unlikely to be a single detachment fault as proposed by Percival et al. (2015, 2017) and Percival and Tschirhart (2017). The sequences Ps1 and Ps2 might not have been deposited where they are absent beneath sequence Ps4, however, this is inconsistent with the lateral continuity of sequence Ps1 in the eastern belts. Therefore, the Archean and Ps1–Ps3 strata were probably eroded prior to progradation of Ps4 across the central Rae Craton.
The intracratonic setting is envisaged as foreland to the northwest-prograding Chesterfield fault zone and possibly reactivated residual western highlands of the Slave-Rae collision zone, during the early post-DP1 (Snowbird), pre-DP2 stage of the Hudsonian orogeny. This model is suggested by the age distribution of the detrital zircon suites that favours provenance from the western and south Rae Craton. The graphitic slate conductors and abundant small cross-cutting sandstone dykes in the turbiditic upper portion of sequence Ps4 in the western Amer to Naujatuuq belts are consistent with sub-wave-base sedimentation during active tectonism, suggesting continued deepening as the prograding DP2 foreland basin migrated into the Amer Belt region. The Ketyet River Belt with its deep marine setting would have been the most proximal foreland basin; it was also the most impacted by the advancing DP2 front from which it was shed, with multiple wedges of coarse breccia and conglomerate resting on Neoarchean rocks beneath DP2 thrusts (Fig. 2).
6 Conclusions
New integrated outcrop, drill core, and geophysical data for the Amer Belt have verified, linked, and extended the first lithostratigraphic schemas and maps of Young (1979), Knox (1980), and Patterson (1980a, b; 1981, 1986). The eight informal formations named by Young (1979) are here proposed as type examples of four revised regional Paleoproterozoic cover sequences limited to the central Rae Craton of the western Churchill Province. Sequence Ps1 is highly deformed quartzite and schistose conglomerate of the Ayagaq Lake formation that unconformably overlies Neoarchean granitic and volcano-sedimentary rocks, after a depositional gap spanning the 2.3 Ga Arrowsmith orogeny that has no record in the study area. Sequence Ps2 comprises black slate to phyllite of the 2126 ± 24 Ma (Re–Os) Resort Lake formation and overlying highly strained dolomitic marble of the Aluminium River formation. Sequence Ps3 comprises undated foliated tholeiitic basalt of the Five Mile Lake formation, grey phyllitic siltstone of the Three Lakes formation, foliated calcareous feldspathic sandstone of the Oora Lake formation, and foliated calcareous feldspathic sandstone of the Showing Lake formation. The contacts between sequences Ps1, Ps2, and Ps3 are gradational but locally highly strained. The unconformably overlying sequence Ps4 comprises pristine, rhythmically interbedded, lithic feldspathic sandstone and mudstone of the <1.90 Ga (detrital zircon) Tahiratuaq group (Young’s Itza Lake formation).
Calhoun et al. (2014) and White et al. (2021, this volume) verified and extended the structural paradigm by Pehrsson et al. (2013b) that distinguishes sequences Ps1–Ps3 from sequence Ps4 in the central Rae Craton. Only the first three sequences were penetratively deformed during DP1 (the ca. 1.9–1.865 Ga Snowbird orogeny). All four sequences were affected by the overlapping DP2 (ca. 1.87–1.81 Ga Hudsonian orogeny). Correlations of the four sequences with other supracrustal belts in the north and south Rae Craton require consideration of this paradigm.
Stratabound metallogeny and mineralogy reinforce the above sequence analysis. The lower Ps2 sequence in the Amer and Ketyet River belts is characterized by nonmagnetic sulfide, carbonate, and hematite iron-formation with local enrichment in one or more of Ag, As, Au, Bi, Cu, Mo, Ni, Pb, Sb, V, and W. Strong electromagnetic conductors in this unit have been drilled beneath the Thelon Formation in search of unconformity-related uranium deposits. The Ps2–Ps3 sequence transition is characterized by foliated basalt and iron rich phyllite with strong and continuous linear aeromagnetic markers caused by disseminated euhedral magnetite. The Showing Lake formation, uppermost in the Ps3 sequence and unique to the Amer Belt, is the main host of numerous sandstone-hosted stratabound occurrences of disseminated pitchblende, euhedral magnetite, and trace chalcopyrite that are coincident with two continuous linear aeromagnetic markers. The pristine Ps4 flysch and molasse sequence lacks stratabound economic mineral occurrences but does have graphitic conductors and at least one uranium occurrence in the Garry Lake belt. Its characteristic multiple linear aeromagnetic markers are weak in the Amer Belt but strong in the Montresor Belt where they are coincident with paleoplacer heavy mineral laminae that include detrital magnetite as well as zircon as young as 1923.8 ± 5.9 Ma (Percival et al. 2017).
Lateral depositional and erosional variations within each Paleoproterozoic belt are complex yet similar from one belt to the next, suggesting that sedimentation was controlled by a combination of local faulting and craton-scale events. Within each belt, facies changes suggest that faulting focused belt-scale deposition and within-belt variations, particularly at the Ps1–Ps2 transition, and during carbonate and basalt accumulations. Lateral variations of sequence Ps4 are mainly evident between different belts rather than within each belt. Multiple fault arrays were reactivated over time, influencing both preservation of Paleoproterozoic strata and the formation of unconformity-related uranium deposits.
Acknowledgements
E. Nutter and R. MacMillan of Westmin Resources employed Grant M. Young and the first author during the 1979 field season and Grant M. Young in 1980. Through a 2009 field trip, Sally Pehrsson introduced the project team to the geology of the region and the structural paradigm, and continues to provide scientific guidance. From 2009 to 2011, eight companies joined the Northeast Thelon Uranium consortium to share geophysical and geological data with universities and the Geological Survey of Canada (Jefferson et al. 2011). Individual industry geologists also participated in yearly field trips and shared logistical support. Garth Drever (Cameco Corporation) shared digitized versions of the 1:50 000 scale maps of the central Amer Belt produced by Young (1979). Lesley Chorlton (GSC) created the ArcGIS geodatabase for this project and populated it with the digitized legacy maps of Grant M. Young and J. Patterson, guided the first author and students in the use of data acquisition technology, collaborated on field data management and legend development, and reviewed an early draft of this paper. Aboriginal Affairs and Northern Development Canada (Karen Costello), Kivalliq Inuit Association (Veronica Tattuinee), the Government of Nunavut (Eric Prosh, Linda Ham and Ronnie Suluk), and the Nunavut Water Board provided guidance. T. Frisch shared colour slide photographs and memories from his 1982 field season in the Montresor belt. D.G.F. Long catalogued and scanned the 1979 colour slide photographs by Grant M. Young used in some figures. The following university and GSC colleagues shared observations and interpretations: Andrey Bekker, Kathryn Bethune, Tony LeCheminant, Dave Lentz, and Judith Patterson. Martin Fowler, Sonya Dehler, Murray Duke, Al Galley, and Cathryn Bjerkelund of the GSC guided the overlapping uranium projects of the Secure Canadian Energy Supply and Geomapping for Energy and Minerals programs. Warner Miles oversaw geophysical contracts. Pierre Keating and Bill Morris contributed geophysical advice. Bianca D’Aoust, Chris Clarke, Beth Hillary, Chris Stieber, Doug Oneschuk, Yask Shelat, Peter Tschirhart, and David Viljoen provided office and field assistance from 2006 to 2012. Ookpik Aviation (Boris Kotelewetz) provided dependable air support contracted through the Polar Continental Shelf Project of Natural Resources Canada. Baker Lake Lodge (Elizabeth and Boris Kotelewetz) provided excellent room, board, and expediting in Baker Lake from 1979 to 2012. Comments by John Percival on work in progress and by Darrel Long on an early draft were constructive. Detailed and authoritative critical comments by Gary Yeo and an anonymous reviewer greatly improved the manuscript and enhanced some economically significant aspects.
References
Alexandre P., Kyser K., Thomas D., Polito P., Marlatt J. 2009. Geochronology of unconformity-related uranium deposits in the Athabasca Basin, Saskatchewan, Canada and their integration in the evolution of the basin. Mineralium Deposita, 44: 41–59.
Anand A., Jefferson C.W. 2017a. Reactivated fault systems and their effects on outcrop patterns of thin-skinned early thrust imbrications in the Kiggavik uranium camp, Nunavut. Geological Survey of Canada, Open File 7895. pp. 9.
Anand A., Jefferson C.W. 2017b. Outcropping and remotely predicted lineaments, faults, fractures and dykes in the Kiggavik uranium camp of Nunavut. Geological Survey of Canada, Open File 7896. pp. 8.
Asael D., François L.H., Reinhard C.T., Rouxel O., Dauphas N., Lyons T.W., et al. 2013. Coupled molybdenum, iron and uranium stable isotopes as oceanic paleoredox proxies during the Paleoproterozoic Shunga Event. Chemical Geology, 362: 193–210.
Ashton K.E., Hartlaub R.P., Bethune K.M., Heaman L.M., Niebergall G. 2013. New depositional age constraints for the Murmac Bay Group of the Southern Rae Province. Precambrian Research, 232: 70–88.
Aspler L.B., Chiarenzelli J.R., Cousens B.L., Valentino D. 1999. Precambrian geology, northern Angikuni Lake, and a transect across the Snowbird tectonic zone, western Angikuni Lake, Northwest Territories (Nunavut). In Current Research; Geological Survey of Canada. 1999-C: 107–118.
Berman R.G., Davis W.J., Pehrsson S. 2007. Collisional Snowbird tectonic zone resurrected: Growth of Laurentia during the 1.9 Ga accretionary phase of the Hudsonian orogeny. Geology, Geological Society of America, 35: 911–914.
Berman R.G., Pehrsson S., Davis W.J., Ryan J.J., Qui H., Ashton K.E. 2013. The Arrowsmith orogeny: geochronological and thermobarometric constraints on its extent and tectonic setting in the Rae Craton, with implications for pre-Nuna supercontinent reconstruction. Precambrian Research, 232: 44–69.
Berman R.G., Sanborn-Barrie M., Rayner N.M., Carson C., Sandeman H.A., Skulski T. 2010. Petrological and in situ SHRIMP geochronological constraints on the tectonometamorphic evolution of the Committee Bay Belt, Rae Province, Nunavut. Precambrian Research, 181: 1–20. Precambrian Research
Bethune K.M., Berman R.G., Rayner N., Ashton K.E. 2013. Structural, petrological and U–Pb SHRIMP geochronological study of the western Beaverlodge domain: implications for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogen in the SW Rae Craton, Canadian Shield. Precambrian Research, 232: 89–118.
Bethune K.M., Hunter R.C., Ashton K.E. 2006. Age and provenance of the Paleoproterozoic Thluicho Lake Group based on detrital zircon SHRIMP geochronology: a record of tectonic activity in the southwest Rae Province, Saskatchewan. In Summary of Investigations 2006. Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2006-4.2, Paper A-3. Vol. 2, pp. 18.
Bolton T.E., Nowlan G.S. 1979. A Late Ordovician Fossil Assemblage from an Outlier north of Aberdeen Lake, District of Keewatin. In Contributions to Canadian Paleontology. Editedby P.J. Griffin. Geological Survey of Canada, Bulletin. Vol. 321, pp. 1–26.
Calhoun L., White J.C., Jefferson C.W., Patterson J., Tschirhart V. 2014. Integrated geodatabase study of the complexly deformed U-hosting Paleoproterozoic Amer Group, Nunavut. Geological Survey of Canada, Scientific Presentation 19. pp. 26.
Chamberlain K.R., Schmitt A.K., Swapp S.M., Harrison T.M., Swoboda-Colberg N., Bleeker W., et al. 2010. In-situ U-Pb micro-baddeleyite dating of mafic rocks: method with examples. Precambrian Research 183: 379–387.
Chambrias G. 1970. Amer Drill Campaign, NWT, Aquitaine Company of Canada Ltd., Calgary, Indian and Northern Affairs Canada, Yellowknife, Assessment Report 019953 (plan and sections of 17 drill holes, assay results in confidential files of the Uranium Resource Assessment Group at the Geological Survey of Canada, Ottawa).
Corrigan D., Pehrsson S., Wodicka N., De Kemp E. 2009. The Palaeoproterozoic Trans-Hudson Orogen: a prototype of modern accretionary processes. In Ancient Orogens and modern analogues. Editedby J.B. Murphy, J.D. Keppie, A.J. Hynes. Geological Society Special Publication, London. Vol. 327, pp. 457–479.
Curtis L., Miller A.R. 1980. Uranium geology in the Amer-Dubawnt-Yathkyed-Baker Lake Lakes Region, Keewatin District, NWT, Canada. In Uranium in the Pine Creek Geosyncline, Australia. Editedby J. Ferguson, A.B. Goleby. Proceedings of the International Uranium Symposium on the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 595–616.
Davis W.J. 2021. U-Pb zircon age data for supracrustal samples from the White Hills Lake to Amer Lake area, Rae Province, Nunavut, Canada. Geological Survey of Canada Open File 8807. pp. 6.
Davis W.J., Ryan J.J., Sandeman H.A., Tella S. 2008. A Paleoproterozoic detrital zircon age for a key conglomeratic horizon within the Rankin Inlet area, Kivalliq region, Nunavut: implications for Archean and Proterozoic evolution of the area. Geological Survey of Canada, Current Research 2008. pp. 8.
Davis W.J., Sanborn-Barrie M., Berman R.G., Pehrsson S. 2021. Timing and provenance of Paleoproterozoic supracrustal rocks in the central Thelon tectonic zone, Canada: implications for the tectonic evolution of western Laurentia from ca. 2.1 to 1.9 Ga. Canadian Journal of Earth Sciences, 58: 378.
Dziawa C., Gaidies F., Percival J. 2019. Conditions and timing of low-pressure – high-temperature metamorphism in the Montresor belt, Rae Province, Nunavut. Canadian Journal of Earth Sciences, 56(6): 654–671.
Frisch T. 2000. Precambrian geology of Ian Calder Lake, Cape Barclay, and part of Darby Lake map areas, south-central Nunavut.Geological Survey of Canada Bulletin, 542: 51(2 sheets).
Fuchs H., Hilger W., Prosser E. 1986. Geology and exploration history of the Lone Gull property. In Uranium Deposits of Canada. Special Volume 33, pp. 286–292. Canadian Institute of Mining and Metallurgy.
Gall Q., Peterson T.D., Donaldson J.A. 1992. A proposed revision of early Proterozoic stratigraphy of the Thelon and Baker Lake basins, Northwest Territories. In Current Research, Part C, Canadian Shield, Geological Survey of Canada Paper 92-1C, pp. 129–137.
Gandhi S.S. 1989. Geology and uranium potential of the Thelon Basin and adjacent basement in comparison with the Athabasca Basin region. In Uranium resources and geology of North America. Editedby E. Muller-Kahle. International Atomic Energy Agency Tech. Doc. 500. pp. 411–428.
Gandhi S.S. 1995. An overview of the exploration history and genesis of Proterozoic uranium deposits in the Canadian Shield, Exploration and Research for Atomic Minerals. Atomic Minerals Division of India, 8: 1–47.
Geological Survey of Canada. 1974. Amer Lake, District of Keewatin, Northwest Territories. Geological Survey of Canada, Geophysical Series Map 7869 G, 1 sheet.
Heywood W.W. 1977. Geology of the Amer Lake Map-Area, District of Keewatin, Report of Activities, Part A, Geological Survey of Canada Paper 77-1A. pp. 409–410.
Hoeve J., Sibbald T. 1978. On the genesis of Rabbit Lake and other unconformity-type uranium deposits in northern Saskatchewan, Canada. Economic Geology, 73: 1450–1473.
Hoffman P.F. 1988. United plates of America, the birth of a craton: early Proterozoic assembly and growth of Laurentia. Annual Review of Earth and Planetary Sciences, 16: 543–603.
Hoffman P.F., Bell I.R., Hildebrand R.S., Thorstad L. 1977. Geology of the Athapuscow Aulacogen, East Arm of Great Slave Lake, District of Mackenzie. Geological Survey of Canada, Report of Activities, Paper 77-1A. pp. 117–129.
Hunter R.C., Bethune K.M., 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. Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003-4.2, CD-ROM, Paper A-2. Vol. 2, pp. 14.
Hunter R.C., Bethune K.M., Yeo G.M., Ashton K.E. 2004. Investigations of the Thluicho Lake Group: a stratigraphic, structural and tectonic perspective (Uranium City Project). In Summary of Investigations 2004. Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004-4.2, CD-ROM, Paper A-9. Vol. 2, pp. 14.
Huyck H.L.O. 1990. When is a metalliferous black shale not a black shale? In Metalliferous black shales and related ore deposits. Proceedings, 1989 United States Working Group Meeting, International Geological Correlation Program. Editedby R.I. Grauch, H.L.O. Huyck. US Geological Survey Circular 1058. pp. 42–56.
Jefferson C.W., Hunter R., McLaren M., Peterson T., Skulski T., Rainbird R., et al. 2011. Northeastern Thelon Basin uranium region: geological compilation for geophysical consortium planning. Geological Survey of Canada, Open File 6950, 1 sheet.
Jefferson C.W., Pehrsson S., Tschirhart V., Peterson T., Chorlton L., Bethune K., et al. Geology and metallogeny of the northeast Thelon Basin region and comparison with the Athabasca Basin. In Rae synthesis, GEM program. Editedby S. Pehrsson, N. Wodicka. Geological Survey of Canada, Bulletin 612, paper 08. In press.
Jefferson C.W., Peterson T., Tschirhart V., Davis W., Scott J.M.J., Reid K., et al. 2013. LIPs and Proterozoic uranium (U) deposits of the Canadian Shield. Geological Survey of Canada, Open File 7352, 56 pages + spread sheet.
Jefferson C.W., Thomas D., Quirt D., Mwenifumbo C.J., Brisbin D. 2007b. Empirical models for Canadian unconformity-associated uranium deposits. In Proceedings of Exploration 07. Editedby B. Milkereit. Fifth Decennial International Conference on Mineral Exploration. pp. 741–769.
Jefferson C.W., Thomas D.J., Gandhi S.S., Ramaekers P., Delaney G., Brisbin D., et al. 2007a. Unconformity-associated uranium deposits of the Athabasca Basin, Saskatchewan and Alberta. In EXTECH IV: Geology and Uranium EXploration TECHnology of the Proterozoic Athabasca Basin, Saskatchewan and Alberta. Editedby C.W. Jefferson, G. Delaney. Geological Survey of Canada, Bulletin 588. pp. 23–68.
Jefferson C.W., White J.C., Young G.M., Patterson J., Tschirhart V.L., Pehrsson S.J., et al. 2015. Outcrop and remote predictive geology of the Amer Belt and basement beside and beneath the northeast Thelon Basin, in parts of NTS 66-A, B, C, F, G and H, Kivalliq Region, Nunavut, Geological Survey of Canada, Open File 7242, 1 sheet.
Kendall B.S., Creaser R.A., Ross G.M., Selby D. 2004. Constraints on the timing of Marinoan “snowball Earth” glaciation by 187Re–187Os dating of a Neoproterozoic, post-glacial black shale in Western Canada. Earth and Planetary Science Letters, 222: 729–740.
Knox A.W. 1980. The geology and uranium mineralization of the Aphebian Amer Group, southwest of Amer Lake, District of Keewatin, NWT. M.Sc. Thesis (unpublished), University of Calgary. pp. 159.
Laporte P.J. 1981. Amer Lake project. In Mineral Industry Report. Northwest Territories East of 104° West longitude, Indian and Northern Affairs Canada, Yellowknife, Report EGS 1981-11. Volume 2 of 3, pp. 50–51.
LaRocque A., Leblon B., Harris J., Jefferson C.W., Tschirhart V., Shelat Y. 2012. Use of multi-beam RADARSAT-2 dual-polarization C-HH and C-HV imagery for surficial geology mapping in Nunavut, Canada. Canadian Journal of Remote Sensing, 38(3): 281–305.
LeCheminant A.N., Ashton K.E, Chiarenzelli J., Donaldson J.A., Best M.A., Tella S., Thompson D.L. 1983. Geology of Aberdeen Lake map area, District of Keewatin: preliminary report. in Current Research, Part A; Geological Survey of Canada Paper 83-1A: 437–448.
LeCheminant A.N., Jackson M.J., Galley A.G., Smith S.L., Donaldson J.A. 1984. Early Proterozoic Amer Group, Beverly Lake map area, District of Keewatin. In Current Research, Part B, Geological Survey of Canada, Paper 84-1B. pp. 159–172.
MacIsaac D.A. 2011. Paleoproterozoic supracrustal deformation, Amer Lake, Nunavut. B.Sc. Thesis (Unpublished), Department of Geology, The University of New Brunswick. pp. 43.
Martin A.P., Prave A.R., Condon D.J., Lepland A., Fallick A.E., Romashkin A.E., et al. 2015. Multiple Palaeoproterozoic carbon burial episodes and excursions. Earth and Planetary Science Letters, 424: 226–236. Elsevier.
McEwan B.J. 2012. Structural style and regional comparison of the Paleoproterozoic Ketyet River group in the region north-northwest of Baker Lake, Nunavut. M.Sc. Thesis, University of Regina, Regina, Saskatchewan. pp. 121.
Miles W., Oneschuk D. 2013. Northeastern Thelon – Garry Lake, NU, aeromagnetic compilation, Parts of NTS 55, 65, 66 and 76, Nunavut. Geological Survey of Canada Open File, 7461, 2 sheets.
Miller A.R. 1980. Uranium geology of the eastern Baker Lake Basin, District of Keewatin, Northwest Territories. Geological Survey of Canada Bulletin, 330: 63.
Miller A.R. 1995. Polymetallic unconformity-related uranium veins in Lower Proterozoic Amer Group, Pelly Lake map area, northern Thelon Basin, Churchill Province, Northwest Territories, in Current Research 1995-C. Geological Survey of Canada. pp. 151–161.
Miller A.R., LeCheminant A.N. 1985. Geology and uranium metallogeny of Proterozoic supracrustal successions, central District of Keewatin, N.W.T. with comparisons to northern Saskatchewan. In Geology of uranium deposits. Edited by T.I.I. Sibbald, W. Petruk. Canadian Institute of Mining and Metallurgy, Special 32. pp.167–185.
Nicholls P.R.J. 2005. Thelon Project, Geology, Mineralization and Exploration Potential, Aberdeen Lake Area, Nunavut Territory, NTS 66 B, G, H (65°00′N; 99°00′W). Report prepared for Ceduna Capital Corporation. pp. 39.
Nutter D.E. 1979. Report on Geology, Geophysics, Geochemistry and Diamond Drilling. Western Mines Ltd, Amer Lake Area, Keewatin, Indian and Northern Affairs Canada, Assessment Report 061829, 1 report. 43 maps.
Patterson J.G. 1980a. Petrographic study of rocks from the Amer Lake are, District of Keewatin. Canada Department of Indian Affairs and Northern Development. Economic Geology Section, EGS report 1980-1.
Patterson J.G. 1980b. Preliminary geology map, eastern end of the Amer Belt, NTS 66H/7, 8, 9, 10. Canada Department of Indian Affairs and Northern Development. Economic Geology Section, EGS report 1980-9.
Patterson J.G., Jefferson C.W., Kjarsgaard B.A., Pehrsson S., McEwan B., Calhoun L., et al. 2012. Differentiation of Neoarchean from Paleoproterozoic basalt in the uraniferous Amer and Whitehills belts, northeast Thelon Region, Nunavut. GAC-MAC Annual Meeting, St. John’s, Abstracts. pp. 103.
Pehrsson S.J., Berman R., Davis W.J. 2013b. Paleoproterozoic orogenesis during Nuna aggregation: a case study of reworking of the Archean Rae Craton, Woodburn Lake, Nunavut.Precambrian Research, 232: 167–188.
Pehrsson S.J., Berman R., Eglington B., Rainbird R. 2013a. Two Neoarchean supercontinents revisited: The case for a Rae family of cratons. Precambrian Research, 232: 27–43.
Pelly D., Nales F. 1986 (updated 23 August 2005). Hanningajuq: Inuit traditional place names, Garry Lake, Pelly Lake region. World Wildlife Fund Canada and Baker Lake Hunters and Trappers Association. NTS 66F, 66 G, 66 J, 66 K, scale 1:1,000,000.
Percival J.A., Tschirhart V. 2017. Trans-Hudsonian far-field deformation effects in the Rae foreland: an integrated geological- 3D magnetic model. Tectonophysics, 699: 82–92.
Percival J.A., Davis B., Petts D., Jackson S., Berman R., Shalchi B., et al. 2019. Rae impact spherules: aftermath of a 2.1 Ga exoplanet strike. In Abstracts, Goldschmidt Conference, Barcelona, 21 August 2019, 04b.
Percival J.A., Davis W.J., Hamilton M.A. 2017. U–Pb zircon geochronology and depositional history of the Montresor group, Rae Province, Nunavut, Canada. Canadian Journal of Earth Sciences, 54: 512–528.
Percival J.A., Tschirhart V., Davis W., Berman R.G., Ford A. 2015. Geology, Montresor River area, Nunavut, parts of NTS 66-H and NTS 66-I. Geological Survey of Canada, Canadian Geoscience Map, 231(preliminary), scale 1:100 000.
Peterson T.D. 1995. A potassic phreatomagmatic volcanic centre in the Thelon Basin: implications for diamond exploration. in Current Research; Geological Survey of Canada 1995-C: 19–26.
Peterson T., Pehrsson S., Jefferson C., Scott J., Rainbird R. 2010. The Dubawnt Supergroup, Canada: a LIP with a LISP, December 2010 LIP of the month. Available from http://www.largeigneousprovinces.org/LOM.html [accessed January 15, 2011].
Peterson T.D., LeCheminant A.N. 1996. Ultrapotassic rocks of the Dubawnt Supergroup, District of Keewatin, N.W.T. In Searching for diamonds in Canada. Edited by A.N. LeCheminant, D.G. Richardson, R.N.W. Dilabio, K.A. Richardson. Geological Survey of Canada, Open File 3228. pp. 97–100.
Peterson T.D., Jefferson C.W., Anand A. 2015b. Geological setting and geochemistry of the ca. 2.6 Ga Snow Island suite in the central Rae Domain of the Western Churchill Province, Nunavut. Geological Survey of Canada, Open File 7841. pp. 29.
Peterson T.D., Scott J.M.J., Jefferson C.W. 2011. Uranium-rich bostonite-carbonatite dykes in Nunavut: recent observations. Geological Survey of Canada, Current Research, 11: 12.
Peterson T.D., Scott J.M.J., LeCheminant A.N., Jefferson C.W., Pehrsson S.J. 2015a. The Kivalliq Igneous Suite: Anorogenic bimodal magmatism at 1.75 Ga in the western Churchill Province, Canada. Precambrian Research, 262: 101–119.
Peterson T.D., van Breemen O., Sandeman H., Cousens B. 2002. Proterozoic (1.85–1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland. Precambrian Research, 119: 73–100.
Peterson T.D., Wodicka N., Pehrsson S.J., Acosta-Gongora P., Tschirhart V., Jefferson C.W., et al. The Rae Province at 2.6 Ga: a Sanukitoid Storm on the Canadian Shield. In Canada’s Northern Shield: new perspectives from the Geoscience for Energy and Minerals Program. Edited by S.J. Pehrsson, N. Wodicka, J. Percival. Geological Survey of Canada, Bulletin 612. In press.
Pilkington M., Tschirhart V. 2017. Practical considerations in the use of edge detectors for geological mapping using magnetic data. Geophysics, 82(3): J1–J8.
Potter E.G., Tschirhart V., Powell J.W., Kelly C.J., Rabiei M., Johnstone D., et al. 2020. Targeted Geoscience Initiative 5: Integrated multidisciplinary studies of unconformity-related uranium deposits from the Patterson Lake corridor, northern Saskatchewan. Geological Survey of Canada Bulletin, 615: 37.
Prasad N., Roscoe S.M. 1996. Evidence of anoxic to oxic atmospheric change during 2.45-2.22 Ga from lower and upper sub-Huronian paleosols, Canada. Catena, 27(2): 105–121. ISSN 0341-8162.
Rainbird, R.H., Hadlari T. 2000. Revised stratigraphy and sedimentology of the Paleoproterozoic Dubawnt Supergroup at the northern margin of Baker Lake Basin, Nunavut. Current Research, Geological Survey of Canada 2000-C8: 1–9.
Rainbird R.H., Davis W.J. 2007. U-Pb detrital zircon geochronology and provenance of the late Paleoproterozoic Dubawnt Supergroup: Linking sedimentation with tectonic reworking of the western Churchill Province, Canada. Geological Society of America Bulletin, 119: 314.
Rainbird R.H., Davis W.J., Pehrsson S.J., Wodicka N., Rayner N., Skulski T. 2010. Early Paleoproterozoic supracrustal assemblages of the Rae domain, Nunavut, Canada: intracratonic basin development during supercontinent break-up and assembly. Precambrian Research, 181: 167–186.
Rainbird R.H., Davis W.J., Stern R.A., Peterson T.D., Smith S.R., Parrish R.R., Hadlari T. 2006. Ar-Ar and U-Pb geochronology of a late Paleoproterozoic rift basin: support for a genetic link with Hudsonian Orogenesis, western Churchill province, Nunavut, Canada. The Journal of Geology, 114: 1–17.
Rainbird R.H., Hadlari T., Aspler L.B., Donaldson J.A., LeCheminant A.N., Peterson T.D. 2003. Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada. Precambrian Research, 125: 21–53.
Rainbird R.H., Stern R.A., Rayner N., Jefferson C.W. 2007. Age, provenance, and regional correlation of the Athabasca Group, Saskatchewan and Alberta, constrained by igneous and detrital zircon geochronology. in EXTECH IV: Geology and uranium EXploration TECHnology of the Proterozoic Athabasca Basin, Saskatchewan and Alberta, (ed.) C.W. Jefferson and G. Delaney; Geological Survey of Canada Bulletin 588: 193–209.
Reid D.F., Walkow W. 1979. Uranerz Exploration and Mining Limited, Amer Lake Area, Keewatin, Indian and Northern Affairs Canada Assessment Report 081003. pp. 19.
Sanborn-Barrie M., Davis W.J., Berman R.G., Rayner N., Skulski T., Sandeman H. 2014. Neoarchean continental crust formation and Paleoproterozoic deformation of the central Rae craton, Committee Bay belt, Nunavut. Canadian Journal of Earth Sciences, 51: 635–667.
Scott J.M.J., Peterson T.D., McCurdy M.W. 2012. U, Th, REE occurrences within Nueltin granite at Nueltin Lake, Nunavut: recent observations, Geological Survey of Canada. Current Research, 1:11.
Scott J.M.J., Peterson T.D., Davis W.J., Jefferson C.W., Cousens B.L. 2015. Petrology and geochronology of Paleoproterozoic intrusive rocks, Kiggavik uranium camp, Nunavut. Canadian Journal of Earth Sciences, 52(7): 495–518.
Selby D., Creaser R.A. 2003. Re-Os geochronology of organic rich sediments: an evaluation of organic matter analysis methods. Chemical Geology, 200: 225–240.
Shawwa N., McLoughlin-Coleman T., Babechuk M., Parkinson A., Rainbird R. 2021. Dynamic environmental change at the cusp of the Great Oxidation Event: the Gowganda-Lorrain Fm. transition, northern Cobalt basin, Ontario. In Geological Association of Canada Annual Meeting, Abstracts with Programs. Vol. 44, pp. 276.
Sheen A., Heaman L., Kjarsgaard B., Ootes L., Pearson G., Creaser R. 2019. Athapuscow aulacogen revisited: Geochronology and geochemistry of the 2046 Ma Union Island Group mafic magmatism, East Arm of Great Slave Lake, Northwest Territories, Canada. Precambrian Research, 321: 85–102.
Shelat Y., Leblon B., La Rocque A., Harris J., Jefferson C., Lentz D., Tschirhart V. 2012a. Effects of incidence angles on mapping accuracy of surficial geology in Umiujalik Lake area, Nunavut, using multi-beam RADARSAT-2 polarimetric images. Part 1. Nonpolarimetic analysis. Canadian Journal of Remote Sensing, 38(3): 383–403.
Shelat Y., Leblon B., La Rocque A., Harris J., Jefferson C., Lentz D., Tschirhart V. 2012b. Effects of incidence angles on mapping accuracy of surficial geology in Umiujalik Lake area, Nunavut using multi-beam RADARSAT-2 polarimetric images. Part 2. Polarimetic analysis. Canadian Journal of Remote Sensing, 38(3): 404–423.
Smith S.L. 1984. Sedimentology of Amer group rocks, District of Keewatin, Northwest Territories. M.Sc. Thesis (Unpublished), Carleton University. pp. 66.
Smoliar M.I., Walker R.J., Morgan J.W. 1996. Re–Os isotope constraints on the age of Group IIA, IIIA, IVA, and IVB iron meteorites. Science, 271: 1099–1102.
Stanton R.A. 1979. Genesis of uranium-copper occurrence 74–1E, Baker Lake, Northwest Territories, Ontario. M.Sc. thesis, University of Western Ontario, London. pp. 167.
Taylor G.C., Cecile M.P., Jefferson C.W., Norford B.S. 1979. Stratigraphy of Ware (east half) map area, southeastern British Columbia. In Current Research, Part A, Geological Survey of Canada Paper 79-1A. pp. 227–231.
Tella S. 1994. Geology, Amer Lake (66H), Deep Rose Lake (66 G), and parts of Pelly Lake (66F), District of Keewatin, Northwest Territories. Geological Survey of Canada Open File 2969. scale 1:250,000.
Tella S., Heywood W.W., Loveridge W.D. 1985. A U-Pb age on zircon from a quartz syenite intrusion, Amer Lake map area, District of Keewatin, NWT. In Current Research, Part B, Geological Survey of Canada Paper, 85-1B. pp. 367–370.
Tella S., Thompson D.L., James D.T. 1984. Geology of parts of the Deep Rose Lake and Pelly Lake map areas, District of Keewatin. In Current Research, Part A, Geological Survey of Canada, Paper 84-1A. pp. 313–322.
Thiessen E.J., Gibson H.D., Regis D., Pehrsson S., Ashley K.T., Smit M.A. 2020. The distinct metamorphic stages and structural styles of the 1.94-1.86 Ga Snowbird Orogen, Northwest Territories, Canada. Journal of Metamorphic Geology, 38: 963–992.
Tippett C.R., Heywood W.W. 1978. Stratigraphy and structure of the northern Amer group (Aphebian), Churchill structural province, District of Keewatin. In Current Research, Part B, Geological Survey of Canada, Paper 78-1B. pp. 7–11.
Tschirhart V. 2013. Geophysical and geological integration and interpretation of the northeast Thelon Basin, Nunavut. Ph.D. Thesis, McMaster University, Hamilton, Ontario. pp. 180.
Tschirhart V., Jefferson C.W., Morris W.A. 2017. Basement geology beneath the northeast Thelon Basin, Nunavut: Insights from integrating new gravity, magnetic and geological data. Geophysical Prospecting, 65(2): 617–636.
Tschirhart V., Morris W.A., Jefferson C.W. 2013b. Faults affecting northeast Thelon Basin: improved basement constraints from source edge processing of aeromagnetic data. In Uranium in Canada: geological environments and exploration developments. Edited by E. Potter, D. Quirt, C.W. Jefferson. Special Issue of Exploration and Mining Geology (CIM). Vol. 21, pp. 105–113.
Tschirhart V., Morris W.A., Jefferson C.W. 2014. Unconformity surface architecture of the northeast Thelon Basin, Nunavut, derived from integration of magnetic source depth estimates. Interpretation, 2(4): SJ117.
Tschirhart V., Morris W.A., Oneschuk D. 2011a. Geophysical series, geophysical compilation project, Thelon Basin, Nunavut, NTS 66A, B, and parts of 65 N, O, P, 66C, F, G and H. Geological Survey of Canada Open File, 6944, 1 sheet.
Tschirhart V., Morris W.A., Jefferson C.W., Keating P., White J.C., Calhoun L. 2013a. 3D geophysical inversions of the northeast Amer Belt and their relationship to the geologic structure. Geophysical Prospecting, 61: 547–560.
Tschirhart V., Morris W.A., Ugalde H., Jefferson C.W. 2011b. Preliminary 3D geophysical modelling of the Aberdeen sub-basin, northeast Thelon Basin region, Nunavut, Geological Survey of Canada. Current Research, 4: 12.
Tschirhart V., Pehrsson S., Card C., Potter E.G., Powell J., Pana D. 2021. Interpretation of buried basement in the southwestern Athabasca Basin, Canada, from integrated geophysical and geological datasets. In Thematic collection: Uranium fluid pathways. Edited by E. Potter, V. Tschirhart, J. Powell. Geochemistry: exploration, environment, analysis. Vol. 21(1): 1–17.
Tschirhart V., Percival J.A., Jefferson C.W. 2015. Geophysical models of the Montresor metasedimentary belt and its environs, central Nunavut, Canadian Journal of Earth Sciences, 52: 833–845.
Valette M., De Souza S., Mercier-Langevin P., Côté-Mantha O., Simard M., Wodicka N., et al. 2020. Lithological and tectonic controls on banded iron formation-associated gold at the Amaruq deposit, Churchill Province, Nunavut, and implications for exploration. In Targeted Geoscience Initiative 5: contributions to the understanding of Canadian Gold Systems. Edited by P. Mercier-Langevin, C.J.M. Lawley, S. Castonguay, Geological Survey of Canada, Open File 8712. pp. 251–266.
van Breemen O., Aspler L.B. 1994. Detrital zircon ages from Nonacho Basin, western Rae Province, Northwest Territories. In Radiogenic Age and Isotopic Studies Report 8. Geological Survey of Canada, Current Research 1997-F. pp. 49–59.
van Breemen O., Peterson T.D., Sandeman H.A. 2005. U-Pb Zircon geochronology and Nd Isotope geochemistry of Proterozoic granitoids in the western Churchill Province; intrusive age pattern and Archean source domains. Canadian Journal of Earth Sciences, 42: 339–377.
White J.C., Calhoun L., Jefferson C.W.(this volume) Paradoxical mid-crustal displacements and stratigraphic continuity: tectonic evolution of the northeastern Paleoproterozoic Amer belt, Nunavut, Canada.
White J.C., Calhoun L., Jefferson C.W. 2021. Paradoxical mid-crustal displacements and stratigraphic continuity: tectonic evolution of the Paleoproterozoic Amer belt, Nunavut, Canada. Geological Association of Canada - Mineralogical Association of Canada. Abstract Volume 44. pp. 307.
Wilton D.H.C., Churchill R.A., Saunders J.K. 1993. Stratigraphy and Metallogeny of the Cod Island and Calm Cove formations, Mugford Group, Northern Labrador. In Geological Survey Branch, Report 93-1, Current Research (1993). Newfoundland Department of Mines and Energy. pp. 91–102.
Yeo G.M. 2005. Stratigraphy and metallogeny of the Paleoproterozoic Thluicho Lake Group, northwestern Saskatchewan (NTS 74N-11). In Summary of Investigations 2005. Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.2, Paper A-2. Vol. 2, pp. 20.
Young G.M. 1979. Report on the geology of the western part of the Amer Belt (NTS Sheets 66G1, G2, H5, H6 and parts of G8 and H4), Keewatin District, Appendix 02 of Mineral Assessment Report 081047 by E Nutter for Western Mines Ltd. (operator), Department of Indian Northern Affairs, Yellowknife. pp. 37, 7 maps.(50000 scale).
Zaleski E. 2005. Geology, Meadowbank River area, Nunavut, Geological Survey of Canada, Map 2068A. scale 1:50 000. Avaiable from https://doi.org/10.4095/220495.
Zaleski E., Pehrsson S. 2005. Geology, Half Way Hills and Whitehills Lake area, Nunavut, Geological Survey of Canada, Map 2069A. scale 1:50 000. Available from https://doi.org/10.4095/220576.
Appendix A. Method and data for Re–Os isochron age (Fig. 15)
Analytical methods
Samples 98RAT-1, 2, 3, 4, and 5 were collected by R. Rainbird from core of Cameco Corporation’s drill hole DPR-10 (Fig. 2). Geochronology was by R. Creaser at the University of Alberta, as follows. From the core samples, all external drilling marks were removed, and the core cut into multiple ∼1 cm thick sections. From each section, several small samples were produced from the inner part of the core, and each sample broken into chips without metal contact. Final crushing into fine powder was performed using an agate mill. A small amount was used initially to determine the Rhenium (Re) concentration, followed by full Rhenium–Osmium (Re–Os) isotope analysis. All Re–Os isotope analyses were carried out by the Carius tube dissolution method using H2SO4 + CrO3 followed by solvent extraction, ion exchange, and Negative Thermal Ion Mass Spectrometry as described in detail by Selby and Creaser (2003) and Kendall et al. (2004). For all samples, Os was analyzed by Electron Multiplier detector, and Re was analyzed by Faraday collector. During analysis of Os and Re, isotopic analysis of Os and Re standard were carried out to ensure accuracy and all results were within the normal tolerances for Os and Re standards. All isochron regressions are calculated using the program Isoplot and all uncertainties are quoted at the 2σ level as produced by Isoplot, using λ187Re = 1.666e−11·a−1 (Smoliar et al. 1996).
Re–Os data.
Appendix B. Major and trace elements data.
Information & Authors
Information
Published In
Canadian Journal of Earth Sciences
Volume 60 • Number 7 • July 2023
Pages: 1005 - 1039
History
Received: 13 July 2022
Accepted: 13 October 2022
Accepted manuscript online: 21 November 2022
Version of record online: 9 February 2023
Notes
This paper is part of a Collection titled “Understanding the Precambrian: a collection of papers in celebration of Grant McAdam Young (1937–2020)”.
Faculty of Science – Earth and Atmospheric Sciences, University of Alberta, 3-18A Earth Sciences Building, 11223 Saskatchewan Drive NW, Edmonton, AB T6G 2E3, Canada
Author Contributions: Formal analysis, Methodology, Writing – original draft, and Writing – review & editing.
The authors declare there are no competing interests.
Metrics & Citations
Metrics
Other Metrics
Citations
Cite As
C.W. Jefferson, R.H. Rainbird, Grant M. Young, J.C. White, V. Tschirhart, and R.A. Creaser. 2023. The Paleoproterozoic Amer supergroup, Amer Fold Belt, Nunavut: stratigraphy, structure, correlations, and uranium metallogeny. Canadian Journal of Earth Sciences.
60(7): 1005-1039. https://doi.org/10.1139/cjes-2022-0077
Export Citations
If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download.
Cited by
1. The Folster Lake Formation, Nunavut: remnant of a littoral to fluvio-deltaic source to the Paleoproterozoic-age Penrhyn basin
Note: This functionality works only for purchases done as a guest. If you already have an account, log in to access the content to which you are entitled.
We use cookies to improve your website experience. To learn about our use of cookies and how you can manage your cookie settings, please see our Cookie Policy.
