Northwest Canadian Shield Research Articles

New constraints on genesis of the polymetallic veins at Port Radium, Great Bear Lake, Northwest Canadian Shield

The historic Eldorado U-Co-Ni-Cu-Ag veins at Port Radium are hosted by one of the arrays of northeast-trending quartz-filled brittle fault zones in the 1875–1855 Ma Great Bear magmatic zone. The veins formed in a lensoid segment of the fault, which underwent repeated reactivation that created subsidiary fractures, tensional openings, and breccia zones that were favorable for precipitation of multiple stages of ore minerals. The timing of ore precipitation is constrained by two swarms of diabase intrusions. The older ca. 1740 Ma Cleaver dikes predate the polymetallic mineralization and deposition of Mesoproterozoic strata of Hornby Bay Group. The ca. 1590 Ma Western Channel sheets postdate the main ore stages in the veins and intrude the lower strata of the Hornby Bay Group.While diabase intrusions constrain ore precipitation to 1740–1590 Ma, in situ U-Pb analyses of fine-grained uraninite record isotopic resetting at 1453 ± 18 Ma. The δ18O compositions of all uraninite stages range from −35.0 to −27.4 per mil, and reflect an overprint by late meteoric fluids. Chondrite-normalized rare earth element data for uraninite produce bell-shaped patterns centered on Sm-Dy, with slight negative Eu anomalies. The age constraints imposed by the dikes, δ18O values and REE signatures of the early U and Co-Ni sulfarsenide stages indicate that the U and base metal mineralization at Port Radium is akin to the basement-hosted, polymetallic unconformity-related U deposits in the coeval Athabasca and Thelon basins. The 1453 Ma isotopic resetting builds to the growing body of evidence for a major event in Laurentia at 1.4 Ga while the youngest Ag-Bi stage at Port Radium was likely related to intrusion of the Western Channel diabase.

A Paleoproterozoic Andean-type iron oxide copper-gold environment, the Great Bear magmatic zone, Northwest Canada

Iron oxide copper-gold (IOCG) and associated iron-oxide apatite (IOA) styles of metallic mineralization are recognized throughout the Paleoproterozoic Great Bear magmatic zone of the northwest Canadian Shield. The Great Bear magmatic zone was constructed between ca. 1876 and 1855Ma on top of the older Hottah terrane, which preserves continental arc magmatism that began around ca. 2.0 to 1.97Ga and continued between ca. 1.93 and 1.89Ga. The Great Bear represents the final stages of ca. 150 million years of intermittent and pulsed magmatism related to an evolving continental orogenic belt. The preserved geology supports a dramatic geodynamic change in the subduction zone process at ca. 1875Ma, a key driving mechanism for magma and metal mobilization, and was rapidly followed by a large-scale introduction of felsic-intermediate plutons. The overall tectonic setting is partially constrained from new and previously published geochemical data that show that the volcanic and plutonic rocks are high-K calc-alkaline to shoshonitic in nature (e.g., high K2O, Th/Yb, and Ce/P205). They also have suprasubduction-zone geochemical signatures, including primitive mantle normalized positive Th and negative Nb, P, and Ti anomalies. The data support the primary melts were derived from a GLOSS-modified mantle wedge. Three-dimensional rendering of geophysical datasets suggest that two (of four) preserved surfaces within the upper mantle lithosphere, at 70 to 120km depths, represent frozen, subducted oceanic slabs, and likely were the drivers for the bulk of Hottah and Great Bear arc magmatism. The older slab is northwest-striking and dips 12° to 15° northeast, whereas the younger is deeper and north-striking, dipping 13° east. The geometry of the surfaces are comparable with 4D modeling, where a subduction zone is temporarily shut down due to plateau collision, and then steps oceanward and re-initiates; there is no need for polarity reversal of the subduction system. This new geometry and the related inferences about process should be the focus of future research in the region, but for the time-being it can be stated that these subduction and collisional processes were the first order control on lithospheric evolution, and therefore metallic mineralization. Overall, the Great Bear magmatic zone IOCG and related mineralization is not comparable to other Proterozoic IOCG belts, such as those in Australia. However, the complexity of mineralization styles, the spatial-temporal relationship between IOA and IOCG mineralization, the suprasubduction zone environment, and a major change in tectonic regime are features similar to Andean-type IOCG mineralization, as well as Cordilleran alkali porphyry Cu-Au deposits. This further establishes the linkages between subduction zone processes and IOCG formation, as well as relationships in the IOCG-porphyry deposit continuum model.

Isotopic provenance of the lower Muskwa assemblage (Mesoproterozoic, Rocky Mountains, British Columbia): new clues to correlation and source areas

The Proterozoic Muskwa assemblage is a thick (6 km) sequence of essentially unmetamorphosed, predominantly fine-grained siliciclastic and carbonate strata carried within thrust sheets of the Canadian Rocky Mountains of northeast British Columbia. It is inferred to be widespread in the subsurface of the autochthon to the east on the basis of seismic reflection profiles. The age and stratigraphic affinities of this succession are uncertain, although it is known to be older than 779 Ma based on the age of cross-cutting dikes. We report U-Pb SHRIMP ages for detrital zircons from four sandstone units and Sm-Nd data for interbedded shales that refine stratigraphic ages and correlation. The provenance characteristics of the Muskwa assemblage offer another piece in the puzzle of correctly assembling Rodinia. 207Pb/ 206Pb ages determined by SHRIMP analysis of 94 detrital zircon grains from four sandstone units range from ca. 3075–1766 Ma with a pronounced concentration of ages at ca. 1850 Ma and a paucity of grains in the interval 1920–2000 Ma. The youngest, most concordant analysis constrains the maximum age of the Muskwa assemblage to 1766±14 Ma (99.5% concordant). Sm-Nd analysis of associated mudrocks result in a range of ε Nd values that overlap with western Canadian Shield sources but result in T DM model ages of 2.30–2.45 and 2.18–2.33 Ga, using Goldstein et al. (1984) or DePaolo (1981) models, respectively. The pattern of U-Pb ages is similar to that of the western Canadian Shield but precludes a significant contribution from Proterozoic orogens in the northwest Canadian Shield, specifically the Thelon, Taltson and Calderian phase of Wopmay orogen. The abundance of grains with young ages (1850–1770 Ma) implies derivation from similarly aged rocks of the Rimbey granite belt, Hearne Province and possibly the internides of the TransHudson Orogen. The provenance characteristics of the Muskwa are compatible with sediment derived from unroofing and erosion of the orogenic topography formed by the collisional shortening of the Hearne Province during terminal assembly of the Canadian Shield ca. 1.8 Ga. The provenance characteristics of the Muskwa assemblage are distinct from those considered typical of Succession B strata, which are characterized by detrital zircons and Sm-Nd signatures derived from the Grenville Province of eastern North America. Hence, the Muskwa assemblage is assigned to Succession A, although it cannot be demonstrated which, if any, of the Succession A successions in western Canada are direct correlatives. Regional considerations suggest that the most likely correlation is with cratonic sand sheets of the Amundsen, Thelon and Athabasca basins that developed immediately after assembly of the core of Laurentia.

Distinction of Marine from Alluvial Facies in the Paleoproterozoic (1.9 Ga) Burnside Formation, Kilohigok Basin, N.W.T., Canada

ABSTRACT The Burnside Formation records the shift from marine-shelf to largely alluvial conditions in the Paleoproterozoic ( 1.9 Ga) Kilohigok foreland basin in the northwest Canadian Shield. The Burnside is a thick (up to 3.5 km), northwest-tapering siliciclastic wedge representing a sandy braid-delta and braided-river system that prograded northwest across the Archean Slave craton. Criteria are developed to distinguish shallow-marine facies from alluvial facies allowing construction of a stratigraphic framework for this unfossiliferous, alluvially dominated succession. The sandy alluvial system prograded over a storm-influenced fine-grained siliciclastic shelf. Shelf associations can be divided into siltstone-dominated and mixed siltstone-sandstone. The siltstone-dominated association comprises facies ranging from mudstones deposited below storm-wave influence to siltstones deposited under storm and fair-weather wave conditions. Mixed siltstone-sandstone facies form coarsening-upward sequences on the scale of several meters to a few tens of meters that appear to represent prograding marine sand bars. Initial alluvial influence is manifested by associations of mixed siltstone and sandstone with shallow-water wave-formed and current-formed structures, abundant soft-sediment deformation, and isolated meter-size channels filled with trough-cross-bedded andstone and associated rip-up clasts. Thick (tens to hundreds of meters) tabular-bedded fine sandstones represent a delta-platform environment between coarser-grained trough-cross-bedded braid-delta distributary channels. Unidirectional paleocurrents in the transitional alluvial facies indicate predominantly west to southwest transport. The bulk of the Burnside comprises several associations representing sandy alluvial braid-plain facies. Conglomeratic lithologies are subordinate to sandy ones and are restricted largely to the proximal (southeastern) part of the basin. Braid-plain facies, occupying hundreds of meters of section, consist mainly of hierarchies of trough cross-bedding on scales of a few meters to many tens of meters. Unidirectional paleocurrents from the main alluvial associations cluster with a single narrow west-northwest mode. Associations of sedimentary facies and paleocurrents indicate that initially the alluvial system was restricted to the proximal part of the basin and drained longitudinally between the orogenic hinterland and a synsedimentary flexural arch created by thrust loading. Contemporaneous shallow-marine-shelf facies were present to the northwest over and beyond the flexural arch. An abrupt shift to transverse drainage and the superposition of medial-braid-plain facies directly on marine-shelf and deltaic facies in the foreland mark an abrupt, unconformable contact representing the progradation of the alluvial system over the cratonic foreland. The area of maximum disparity between marine and nonmarine facies coincides with the area of greatest stratigraphic thinning and erosion, and marks th crest of the flexural arch. Only the transverse paleocurrent mode is recorded over the flexural arch and in the distal foreland. Stratigraphic relations suggest that marginal-marine and braid-delta facies tracts in the distal foreland shifted abruptly over large distances and were sensitive to changes in relative sea level. In contrast, braid-plain facies tracts shifted very little, and more gradually. The large lateral extent of alluvial facies suggests that the source area had a humid or monsoon climate.

Temporal distribution of granitoid plutonic rocks in the Archean Slave Province, northwest Canadian Shield

Granitoid rocks in the Slave Province consist of 4.0–2.8 Ga granitoid gneisses that predate the 2.71–2.65 Ga volcanic and turbiditic rocks of the Yellowknife Supergroup and 2.70–2.58 Ga granitoid plutons that intrude the Yellowknife Supergroup. U–Pb zircon ages and Nd and Pb isotopic data indicate that the older granitoids are restricted to the western part of the Slave Province. Granitoid gneisses in the eastern Slave Province, previously suspected to predate the Yellowknife Supergroup, are similar in age to the volcanic rocks.In this paper, the results of a detailed geochronological study of plutonic rocks of the Contwoyto Lake – Nose Lake area of the northcentral Slave Province are reviewed and integrated with the available age data base for plutonic rocks elsewhere in the Slave Province. The data indicate that the timing of the later (<2.7 Ga) plutonism is bimodal, consisting of two distinct magmatic periods separated by approximately 20 Ma of apparent magmatic quiescence. The first period was synvolcanic, dating from 2695 to 2650 Ma. The second period, during which more than 80% of the granitoids presently exposed in the Slave Province were intruded, lasted from 2625 to 2580 Ma, spanning the major Archean deformation events, Syndeformation granitoids, with ages between 2625 and 2595 Ma, are dominantly diorite and tonalité in composition. Late- to post-deformation granitoid rocks, with ages between 2605 and 2580 Ma, range in composition from megacrystic biotite granodiorite to two-mica granite. In general terms, the compositions of the granitoid plutons vary in time from dominantly metaluminous to more strongly peraluminous.Present data show no obvious regional age variation among the younger granitoid rocks across the province. This apparent absence of diachroneity has important implications for models interpreting the magmatism as having evolved in a continental-margin setting, because the extent of contemporaneous plutonism is more than 400 km across strike, considerably broader than in most Mesozoic continental-margin batholiths.

Implications of new chronostratigraphy for tectonic evolution of Wopmay Orogen, Northwest Canadian Shield

Implications of new chronostratigraphy for tectonic evolution of Wopmay Orogen, Northwest Canadian Shield

Post-collisional oblique convergence along the Thelon Tectonic Zone, north of the Bathurst Fault, NWT, Canada

The Thelon Tectonic Zone (TTZ) separates the Slave from the Churchill (Rae) structural provinces of the northwest Canadian Shield. Along the TTZ in the Tinney Hills-Overby Lake area, collision was initiated with northwestward overthrusting of granulite-facies gneiss onto the Slave province accompanied by thinskinned shortening of the Proterozoic cover. In the succeeding transpressive stage, shortening structures assumed a SE-, rather than NW-, vergence; a deformation-front migrated westward into the Slave craton-edge, causing thick-skinned folding of the cover, probably including the detachment; and transpression was partitioned, in craton edge and allochthon, into zones of shortening, shortening + strike-slip, and strike-slip. Ductile deformation along the trace of the late NW-striking brittle Bathurst Fault occurred at this time; this zone links dextral NNE-trending transpressive zones, in which the shortening structures have opposite vergence (SE north of the fault, and NW to the south). The last episode of convergence, tectonic escape, may have been accomplished during brittle deformation on the Bathurst Fault. In the Slave Foreland and the reworked Slave craton-edge, the role of heterogeneous basement deformation is very important. The basement had a complex Archean structure, which was overprinted by systems of Proterozoic shear zones. The bulk strains accomplished by these are expressed as upright folds in the stratiform cover, causing characteristic synclines.

Initiation of cataclastic flow in a mylonite zone

At the mylonite-cataclasite transition in the homogeneous quartzo-feldspathic mylonites of the Great Slave Lake shear zone, northwest Canadian Shield, arrays of distributed macroscopic fractures evolved into zones of cataclastic flow. Fracture arrays occupy elliptical areas (metres by 10s of centimetres) aligned in the mylonite foliation plane. Fracture segments rotated either forwards or backwards in response to deformation. Where fractures rotated backwards, they remained planar and cataclasite did not develop. In the case of forward-rotation, only the central part of each fracture rotated. The rotated fracture segments are notably shorter and more closely spaced (1–2 mm) than those which have not rotated and the fracture array has a sigmoidal geometry. Cataclasite developed by the further deformation of such sigmoidal, forward-rotated fracture arrays. Back-rotated fractures were initially planar, regularly spaced (2–10+ cm) and oriented at 70° to the mylonite foliation; they correspond to extensional faults. Forward-rotated fractures were initially planar, regularly spaced (2 cm) and oriented perpendicular to the mylonite foliation; they do not correspond to classical arrays of extensional or contractional faults. Both the initial orientation of the fractures and their subsequent rotational behaviour were constrained by the ability/inability of the wallrock to extend and by the ability/inability of the fractures to dilate. Within the forward-rotated fracture arrays, cataclasite was produced after the rotated fracture segments had locked. It is proposed that a second perpendicular fracture set cut the earlier sigmoidal fractures resulting in stubby fragments of mylonite. The angular fragments acted as stress raisers within the otherwise homogeneous mylonite. This allowed the transition from distributed macroscopic fracture to impingement-induced localized microfracture and reduction of the fragment size with further strain. When a sufficient volume of interconnected fine material had formed, significant displacements could be accommodated by cataclastic flow, leading eventually to the development of thick (1–100s m) zones of cataclasite. Models of cataclasis developed for porous and carbonate media predict initial grain-scale micro-fracture, slip and fragment rotation leading to frictional wear. The observations made in the Great Slave Lake shear zone suggest that such models may not be directly applicable to the initiation of cataclastic flow developed within non-porous, homogeneous, quartzo-feldspathic mylonite belts in crustal-scale fault zones, at the elevated confining pressures typical of the transition from the plastic to the brittle deformation regimes.

Geochemistry and origin of the Regan Intrusive Suite and other granitoids in the northeastern Slave Province, northwest Canadian Shield

The Regan Intrusive Suite of about 100 plutons of tonalite, granodiorite, and quartz diorite intruded the Yellowknife Supergroup and migmatite terrain in the northwest Slave Structural Province 2.59 Ga ago. Rare-earth-element (REE), trace-element, and major-element analyses from 39 representative whole rocks from the suite suggest it was derived by batch melting of the crust, producing a parental magma of tonalitic or granodioritic composition. By analysing REE from different parts of a zoned pluton, it was concluded that REE distribution was controlled by early separation of quartz diorite from the parent magma by flow differentiation and that the bulk of the REE were contained in early, cumulate, accessory apatite and monazite. The residual magma was further fractionated in pipelike magma chambers during ascent into more leucocratic rocks. Chondrite-normalized REE patterns of single-lithology plutons are similar to lithologies in zoned plutons, and it is proposed they initially segregated during ascent. It was found that granites, which were formerly grouped with the suite, formed in three ways, only one of which is related to the Regan Intrusive Suite.Study of 2.67 Ga old synvolcanic tonalite pluton revealed a strong covariance of light REE with those of the bimodal, calc-alkaline Hackett River Group of volcanic rocks. The data imply a common crustal source, but mass balance requires larger volumes of felsic volcanic rocks than are presently preserved, suggesting that much of the erupted felsic pyroclastic rocks were eroded. Partial melts from synvolcanic tonalite during subsequent regional metamorphism differentially depleted host rocks in REE and concentrated Eu and heavy rare-earth elements (HREE) in trondhjemite pegmatites.

Paleoclimatic Significance of Widespread Proterozoic Silcretes in the Bear and Churchill Provinces of the Northwestern Canadian Shield

ABSTRACT Silcrete is widespread in the Thelon Basin and lower Coppermine Homocline, two Early to Middle Proterozoic cratonic cover sequences in the northwest Canadian Shield. In the Thelon Basin, silcrete occurs as silicified lateritic weathering profiles, weathered tops of rhyolite flows, and silicified terrestrial volcaniclastic sediments. In Coppermine Homocline, silcrete comprises silica-cemented rock fans at the basal unconformity, silicified evaporitic fracture-fills in basement, and silicified ephemeral lake sediments. Petrographic studies of the silcretes indicate that they are strikingly similar to Cenozoic silcretes from southern Africa and Australia formed under arid climates. The silcretes in both Proterozoic sequences are associated with climate-sensitive sediments, such as eolian tes and evaporites, that also suggest an arid environment of deposition. Geochronologic data indicate that the silcretes represent part of an arid paleogeographic province that existed in the northwest Canadian Shield at about 1.85-1.7 Ga. Although the Proterozoic silcretes are petrographically similar to Cenozoic arid climate analogs, the lack of land plants in the Precambrian suggests that geochemical criteria used to distinguish Cenozoic arid and humid climate silcretes may not be applicable to Precambrian examples.

The geochronology of the granitic rocks along the Bear–Slave Structural Province boundary, northwest Canadian Shield

Archean granitic rocks along the southern Bear–Slave boundary fall into three age groups: the oldest are 3000 Ma old intrusive tonalites and granodiorites that form the basement to the Yellowknife Supergroup; the second are syn-volcanic granitic intrusions of ~ 2700 Ma; and the youngest are ~ 2560 Ma granitic and migmatitic diapirs formed in part from supracrustal and granitic rocks. Two Proterozoic thermal events are recognized within the Slave Province. A ~ 2300 Ma event may be related to early rift breakup of the Archean crust and is recorded in Rb–Sr whole-rock and K–Ar mineral systems. A ~ 1970 Ma event was less intense but may be related to further rifting of the Archean and to fault-block depression of the Indin Lake supracrustal basin, the intrusion of a group of granodioritic stocks, and the formation or granitic pegmatite.Within the Bear Province, evidence of a ~ 2700 Ma intrusive event and a ~ 2300 Ma thermal event are preserved in Rb–Sr whole rock systems. Practically all the granitic rocks of the Bear Province, including the Hepburn batholitic rocks, are thought to have been derived wholly or partly from Archean rocks. The main period of Hudsonian deformation and metamorphism was accompanied by a diapiric remobilization of the Archean basement about 1800 Ma ago. Twelve Rb–Sr isochrons, as well as other published geochronologic data from the region, support these conclusions.