Slave craton
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The Slave craton is a relatively small Archean craton (about 300,000 km²) located in the Northwest Territories of Canada. It is dominated by ca. 2.73-2.63 Ga greenstones and turbidite sequences and ca. 2.72-2.58 Ga plutonic rock, with large parts of the craton underlain by older gneiss and granitoid units. The Slave craton is a major component of the Canadian Shield and one of the most distinct and oldest building blocks of the North American cratonic lithosphere. The cratonic block extends from the Great Slave Lake at about 61 °N to Coronation Gulf on the Arctic Ocean at 69 °N. It covers longitudinally the area between about 105 °W to 117 °W. The Slave includes the Acasta Gneiss which is one of the oldest dated rock units on Earth at 4.03 Ga.
The crust of the Slave craton is thought have amalgamated during a 2.69 Ga collision between a proto-Slave western basement complex, known as the Central Slave Basement Complex, and an eastern putative island arc terrane (Hackett River) along a N-S suture. Along the Acasta River, this basement complex yields protolith ages up to ca. 4.03 Ga, but apart from a central core with sporadic ages >3.5 Ga (Acasta to Point Lake), the Central Slave Basement Complex is mostly younger. Primary evidence for this lies in the N-S Pb and Nd isotope boundaries, and the apparent lack of old basement in the eastern Slave craton. From the several hundreds of kimberlites now known across the craton, the following ages have been recorded: Cambrian, Siluro-Ordovician, Permian, Jurassic, Cretacous, and Eocene. (Bleeker et al., 2004)
[edit] Origins of the proto-craton and continent formations
Zircon crystals from the Acasta Gneiss have been radiometrically dated at 4.4 billion years. Rocks this old are thought to have been formed by mantle mafic-ultramafic and crustal acid magmatism. Parts of the Central Slave Basement Complex reveal quartzite gneiss that is similar to the quartzite found in the Southern Cross Province of the Yilgarn craton. A potential supercontinent predating 2.8 Ga is cautiously hypothesized. However, the variable age uncertainties of these Archean quartzite gneisses most probably reflect an overall distribution of a global 3.0-2.8 Ga quartzite peak, rather than a direct correlation between the two cratons. This peak likely reflects progressive growth and stabilization of continental crust toward the end of the Archaean. (Bleeker, 2000).
The Slave craton's amalgamation with the Rae craton (ca. 2.0 Ga) initiated the growth of Laurentia (2.0-1.8 Ga). Within a broader context, it originated from the breakup of the much larger late Archean parental supercraton Sclavia, and is a remnant of the supercontinent Nuna. As a mere fragment of ancient crust, surrounded by Paleoproterozoic rifted margins, it is also a remnant from the break-up of a much larger late Archean landmass of the supercontinent Kenorland. The late Archean and earliest Proterozoic development of Slave crust should thus be viewed in the context of this larger supercraton (Sclavia), even though the shape and size of this earlier supercraton is unknown. The salient point is that cratons like the Slave only preserve parts of the much larger tectonic systems in which they were generated.
Post-dating the assembly of Laurentia and Nuna, the Slave craton, particularly along its margins, became partially buried beneath intra-continental Proterozoic basins. At ca. 1.269-1.267 Ga, the craton was partly uplifted and intruded by the giant Mackenzie dyke swarm, radiating from a plume center west of Victoria Island. This is the last major event affecting the core of the craton, although some younger mafic magmatic events affect its edges (e.g. the ca. 780 Ma Hottah sheets). Since that time, Slave crust has been "bobbing" gently up and down, with interior seas expanding and receding across the craton. Ordovician and Cretaceous sedimentary rocks and fossils are known as wall rock fragments in some of the central Slave kimberlites. (Bleeker, 2006).
Despite the relative stability at the surface, melting events were triggered in the subcontinental mantle lithosphere, leaving their traces as clusters of kimberlites across the craton. From the several hundreds of kimberlites now known across the craton, the following ages have been recorded: Cambrian, Siluro-Ordovician, Permian, Jurassic, Cretacous, and the Eocene. (Bleeker, 2006).
[edit] Early Archean Central Slave Basement Complex
The central and western parts of the craton are underlain by a large Mesoarchean to Hadean basement: the Central Slave Basement Complex (>100,000 km²). Along the Acasta River, this basement complex consists of polymetamorphic gneisses of tonalitic to gabbroic composition that yield protolith ages up to ca. 4.03 Ga. Apart from a central core with sporadic ages >3.5 Ga (Acasta to Point Lake), the Central Slave Basement Complex is mostly younger with important age modes, from detrital and protolith U-Pb zircon ages, around 3.40 Ga, 3.15 Ga, 2.95 Ga and 2.826 Ga. Data from the mantle suggest that at least part of the lithospheric mantle below the central part of the craton may be of similar antiquity. Pre-2.9 Ga supracrustal rocks have been found at the base of some greenstone belts but form only a small component. (Bleeker, 2004). Sometime after 2.85 Ga, the nucleus of the Meso- and Paleoarchean sialic crust of the Central Slave Basement Complex experienced regional uplift followed by gradual subsidence giving rise, first, to craton-scale unconformity, then to the thin transgressive veneer of crossbedded, highly mature, quartzitic sediments. This uplift-subsidence cycle is attributed to a mantle plume that gave rise to a subaerial komatiite flow, now preserved in the detrital chromite grains in the quartzite units. Following this uplift-subsidence cycle, rifting of the thermally weakened crust resulted in voluminous outpouring of pillow basalt flows and intrusion of associated dyke swarms. Much of the craton was submerged at this time so there is little sedimentary evidence left of this rifting. (Bleeker et al., 2000).
[edit] Neoarchean basement complex and Central Slave Cover Group
By at least 2.9 Ga, the basement complex formed a thin but widespread, ca. 2.9-2.8 Ga cover sequence of siliciclastic and sedimentary rocks <200 m. thick. The basement-cover contact represents a sheared and metamorphosed unconformity. The overall topology of the greenstone belts in the western and central Slave province permits all quartzite and banded iron formations to be correlated into a single cover sequence. This lateral correlation is supported by the lithostratigraphic similarities that exist over distances of several hundred kilometers. The most distinctive lithologies are the detrital-chromite-bearing fuchsitic quartzite and thin banded iron formations that mark the onset of the Neoarchean cycle of supracrustal development.
The supermature and commonly fuchsitic quartzites mark the emergence and erosional unroofing of the basement complex in what was probably a CO2-rich atmosphere. Abundant detrital chromite suggest contemporaneous komatiitic volcanism. Similar fuchsitic quartzite sequences occur in many other cratons worldwide, particularly between ca. 3.1 Ga and 2.8 Ga. All the fuchsite in the quartzite units are considered to be the product of mica-producing metamorphic reactions during the shearing and considered to reflect detrital chromite input. In many localities fuchsitic orthoquartzite are intercalated with other clastic rocks such as polymict conglomerate and impure, mica-rich, flaggy quartzite. Pelitic schist occurs between the quartzite rocks and banded iron formation suggesting a progressive deepening of the depositional environment and trangressive migration of the shoreline. In most cases, the banded iron formations are capped with a thin sulfidic chert layer and then abruptly overlain with massive to pillowed basaltic flows. Highly magnesian mafic/ultramafic lavas have been observed at or near the base of the pillow basalt sequence (Bleeker et al., 2000). After 2.4 Ga, mature quartzites are rarely fuchsitic. The cover sequence itself is overlain by 2.73 to 2.63 Ga greenstone belts.
The Neoarchean supracrustal sequence is heavily intruded and cannibalized by plutonic suites ranging in age from 2.72-2.670 Ga synvolcanic plutons to 2.59-2.58 Ga late-orogenic batholithic granites. The supracrustal sequences, collectively known as the Yellowknife Supergroup, are represented by an early cover sequence comprising quartzite and banded iron formation (ca. 2.80 Ga), a thick dominantly tholeiitic greenstone sequence (ca. 2.70 Ga), younger arc-like sequences (ca. 2.69-2.61 Ga), extensive turbidite blankets (ca. 2.68-2.62 Ga), and finally syn-orogenic conglomerates deposited at ca. 2.60 Ga or shortly thereafter. The early cover sequence and the overlying tholeiites represent subaerial exposure and then volcanic-dominated rifting of the basement. Arc-like sequences formed in part on top of the attenuated basement and in progressively widening, juvenile, back-arc-like basins and contain some of Canada's largest undeveloped volcanogenic massive sulfide deposits. After 2.68 Ga, much of the Slave craton became overlain by the Burwash Basin, one of the largest and best-preserved Archean turbidite basins in the world, comparable in size and setting to the Japan Sea. During orogenesis, the supracrustal sequences were telescoped, thickened, and multiply folded between ca. 2.65 and 2.58 Ga, with a peak in crustal anatexis between 2.59-2.58 ga (the "granite bloom"). Numerous orogenic gold deposits formed throughout the Slave craton, either as shear- or vein-hosted deposits in deformed greenstones or within the chemical traps provided by banded iron formations in the turbidites. (Bleeker, 2004).
The Central Slave Cover Group hosts some of the more prominent banded iron formations (BIF) of the Slave craton, although most are thin (1-10 m) and variable in composition along strike, changing from oxide iron formation into silicate-rich varieties or merely ferruginous chert. Locally, however, folding has thickened the highly magnetic BIF into substantial thicknesses (e.g., at Amacher Lake, on the eastern flank of the Sleepy Dragon Complex), resulting in some of the highest amplitude total field magnetic anomalies in the Slave craton. (Bleeker, 2006).
[edit] Ca. 2.73-2.66 Ga volcanism
Wherever the thin cover sequence is recognized, it is overlain by a thick and extensive sequence of tholeiitic basalts, with minor komatiite and rhyolite tuff intercalations. In the Yellowknife greenstone belt, this basalt-dominated volcanic sequence is known as the Kam Group. Possible correlative basalt successions are known across the basement domain, as far east as the Courageous Lake belt, and at least as far north as around the Exmouth antiform in the Acasta area. This basalt sequence typically consists of several hundred meters to several kilometres of pillowed and massive flows, with thin felsic horizons, and intruded by numerous dykes and sills of several generations. (Bleeker, 2006).
Although these lavas extruded in a subaqueous environment, regional correlations suggest a basalt sequence approaching LIP proportions (areal distribution >100,000 km², typical thickness 1-6 km). Stratigraphy, dense dyke swarms, and isotopic data link this basalt sequence to the basement. Well-dated components of this widespread basalt-dominated sequence yield ages from 2.734 Ga to 2.697 Ga. The widespread basaltic volcanism probably accompanied rifting of the basement complex, possibly assisted by mantle plume activity. From a mantle perspective, it seems inconceivable that events associated with the voluminous basaltic volcanism recorded across the ancient basement terrain did not involve thinning or at least modification of the lithospheric mantle below the Central Slave Basement Complex. Large-scale melting was probably triggered by adiabatic rise of asthenospheric mantle. Perhaps, then, the ca. 2.7 Ga basaltic volcanism may have contributed to the highly depleted mantle compositions underlying the core of the craton. (Bleeker, 2006).
[edit] Post-2.70 Ga volcanism
Following ca. 2.7 Ga basaltic volcanism and rifting, most areas in the Slave craton show a transition to calc-alkaline volcanism characterized by abundant felsic and intermediate volcanic rocks, calc-alkaline basaltic rocks, and intercalated volcaniclastic sedimentary rocks. In nearly all areas, these arc-like rocks are stratigraphically overlain by turbiditic sedimentary rocks. Ages for the arc-like volcanic rocks are typically in the range of 2.69-2.66 Ga.
The ca. 2.7 Ga rifting and basaltic volcanism initiated a complex sequence of events including craton-wide, 2.69-2.66 Ga, typically calc-alkalic volcanism and sub-volcanic intrusive activity. These largely juvenile arc-like rocks dominate the eastern part of the craton, while in the central and western part they stratigraphically overlie basement and its basaltic cover. Hence, the most likely setting appears an arc- or back arc-like environment that was constructed on highly extended continental crust. The craton-scale stratigraphic relationships and lack of a suture do not easily support models that invoke collision of an exotic, juvenile arc terrane. (Bleeker, 2006).
[edit] Ca. 2.68-2.66 Ga sedimentation
Starting at ca. 2680 Ma, a broad turbidite basin-the Burwash Basin-developed across much of the craton and progressively buried the volcanic substrate. A persistence of volcanic intercalations up-section and late mafic sill complexes suggest a volcanically active extensional setting, perhaps best compared with modern back-arcs. The minimum size of this basin was ca. 400x800 km, comparable to that of the Japan Sea, and making it the largest and possibly best-preserved Archean turbidite basin in the world. Like the Japan Sea, the Burwash Basin was largely ensialic, in agreement with inferences by early workers.
The Burwash Basin fill consists largely of immature greywackes and mudstones, deposited below wave base, and may locally be up to 10 km thick. Intercalated tuff layers have been dated at ca. 2.661 Ga. Across the Slave craton, the greywacke turbidites have been given different formational names: the classical Burwash Formation in the Yellowknife Domain; the Contwoyto Formation in central and northern Slave, identical in essentially all aspects to the Burwash Formation further south, except for the presence of intercalated iron formations; the Itchen Formation, a more mud-rich facies in the north-central Slave; and the Beechey Lake Group in the northeastern Slave.
Many of the turbidite beds, particularly those of the Burwash and Contwoyto Formation, are sand dominated with only thin silt to mud sections at the top of the graded beds. In the Yellowknife Domain, thick amalgamated sand beds (2-10 m) are not unusual. Petrography, detrital zircons, and geochemical analysis indicate that the greywacke detritus consists of a mixture of mafic and felsic volcanic rocks and uplifted plutonic infrastructure, with only minor input from ancient basement rocks. The main axis of the basin and subsequent structural trends appear to have been northeast-southwest, distinctly across the north-south isotopic boundaries that track the nature of deep basement. This interpretation is based on the following observations:
Identical Burwash Formation turbidites extend from the near Yellowknife area to the northeastern Slave. Banded iron formations in the turbidites are restricted to the northwest half of the craton, suggesting a northeast-southwest facies boundary or tectonic trend across the basin. Earliest folds in the turbidites, which formed between 2.65-2.63 Ga, have northeasterly trends after qualitative "unfolding" of younger fold generations. Early folds appear to form a systematic northeast-southwest trending fold belt. The earliest plutonic suite that intrudes folded Burwash strata, the ca. 2.63 Ga Defeat Suite, appears to form a northeast-southwest-trending magmatic belt across the southeastern half of the craton. With more and better U-Pb zircon ages, a tentative "volcanic line" of 2.661 Ga felsic volcanic complexes, coeval with turbidite sedimentation, has begun to emerge. This volcanic line also trends northeast-southwest and may represent the first recognition of a linear arc system. (Bleeker, 2006).
[edit] Ca. 2.65-2.63 Ga closure of the Burwash Basin
Subsequent tectonic events record the closure and folding of the Burwash Basin across the southern Slave craton with turbidite sedimentation sometime before 2.65 Ga. The latter age constraint is provided by early plutons of the Defeat Suite, a distinct and possibly subduction-related magmatic suite across the southern-southeastern Slave craton. Closure of the highly extended, but largely ensialic back-arc basin allowed considerable shortening and mobility but with a structural style dominated, at least at high structural levels, by fairly systematic, mostly upright, northeast-southwest trending fold trains. At deeper levels, e.g. along the basement-cover interface, the fold trains must have been detached allowing differential shortening of the basement and cover.
The folded Burwash strata do not represent an outboard accretionary prism (which would require a trench setting rather than the more likely back-arc setting), and there is no evidence for a discrete "Contwoyto terrane." The northeast-southwest structural grain of the fold belt is also recognized in the lithospheric mantle. Shallow subduction may have emplaced distinct mantle slabs. These processes terminated with docking of an outboard terrane, either in the southeast or the northwest; but this terrane is not preserved, however, within the exposed Slave craton. Crustal thickening led to uplift and erosional exhumation of folded Burwash strata and the unroofing of Defeat Suite plutons. Detrital zircons of Defeat Suite age are recorded in younger sedimentary packages. (Bleeker, 2006).
[edit] Ca. 2.63 Ga turbidites
2.63 Ga turbidites exist along the northwestern margin of the craton and record a migration of tectonic activity to the northwest. Deposition was coeval with uplift and erosional unroofing of Defeat plutons and the tightly folded Burwash Formation strata. Shortly following their deposition, these younger turbidites were shortened and intruded by ca. 2.616-2.609 Ga tonalite-granodiorite plutons of the Concession suite. (Bleeker, 2004).
In the multiply folded, metamorphosed, and intermittently exposed terrain of the western Slave craton, it has proven difficult to distinguish these younger turbiditic greywackes from Burwash Basin turbidites. There is no sharply defined demarcation line that separates the two turbidite packages and recognition of the younger sequence relies largely on the absence of Defeat Suite-age plutons and the presence of <2.64 Ga detrital zircons. Preliminary work suggests that the younger turbidite sequence contains abundant intercalated iron formations, mostly of silicate facies, those of the Damoti Lake area representing one of the more significant examples. Many of the iron formations are "lean", comprising background turbiditic greywacke variably enriched in metamorphic garnet, other Fe-rich silicates, and/or disseminated sulphides.
A distinct, post-Burwash Basin, greywacke and/or volcaniclastic sediment package, associated with felsic volcanic rocks and subvolcanic intrusions, and dated at approximately 2.716-2.712 Ga, occurs along the tightly folded synclinal core of the High Lake greenstone belt of the northern Slave craton. This package is of significance in that it is one of the few examples of a preserved volcano-sedimentary carapace to one of the major plutonic suites, i.e. the coeval Concession Suite. (Bleeker, 2006).
[edit] Ca. 2.60-2.59 Ga final orogeny
Starting at ca. 2.6 Ga, the entire craton was affected by cross-folding and significant further shortening, characterized by broadly north-south structural trends, and probably in response to final collision along a distant active margin of Sclavia. Moderate overthickening of the crust led to HT-LP metamorphism, widespread anatexis, the appearance of S-type granites, and a hot and weak lower crust, culminating in ca. 2.59-2.58 Ga extension and the regional “granite bloom”. The intrusion of carbonatites and involvement of other mantle-derived melts indicate a role for mantle processes (possible delamination). While peak temperatures were attained in the lower crust, large basement-cored domes were amplified by buoyancy driven deformation; lower crustal devolatilization reactions mobilized gold-bearing fluids; and syn-orogenic clastic basins formed and were immediately infolded into tight synclines. At least one of these syn-orogenic clastic basins formed as late as ca. 2.58 Ga. Late strike-slip faulting overprinted and truncated the synclinally infolded clastic basins. This event in the craton's evolution transferred, irreversibly, a significant fraction of heat-producing elements and lower crustal fluids to the upper crust, thus allowing cooling and stiffening of the lower crust and setting the stage for cratonization and long-term preservation. When the lower crust cooled and coupled with the mantle, the Slave (within Sclavia) finally became a craton. (Bleeker, 2004).
[edit] Northern Slave province, Acasta Gneiss and Point Lake greenstone belt
The Acasta Gneiss is a rock outcrop of Archaean tonalite gneiss located in the western section of the northern Slave province, just west of the Emile River. The rock exposed in the outcrop formed at 4.03 Ga and is the earliest known rock to have survived intact from the Earth's early crust. Plutonic activity occurred in the underlying basement of the Acasta area at ca. 3.4 Ga. and the overlying quartzite and banded iron formation of the Acasta Gneiss Complex ar dated at 2.8 Ga. This cover sequence is conformably overlain by pillow basalt flows, and in turn by metaturbidite units. This conformably overlying strata is part of the Archaen Yellowknife Supergroup. However, as part of the Central Slave Cover Group, the Acasta gneisses are unconformably overlain by the fuchsitic quartzite, firmly placing the Earth's oldest rocks within the old cratonic nucleus of the Slave Province prior to deposition of the ca. 2.8 Ga quartzite units. The ca. 4.0 Ga rocks of the Acasta River area forms an ancient enclave in a polycyclic heterogeneous gneiss terrane that trends around the Emile River sycline east to the Point Lake gneiss domain.
To the east of the Acasta Gneiss lies the Point Lake greenstone belt. Quartzite and a banded iron formation sequence is found beneath the belt and below a small outlying greenstone belt to the west of Point Lake near the Greenville Lake basement complex. Gneissic rocks in the Point Lake area are separated from the gneisses in the Acasta area to the west by the Emile River supracrustal belt. The Emile River belt comprises two marginal greenstone belts of deformed pillowed basalt sequences. (Bleeker et al., 2000).
[edit] Eastern Slave province, Back River Volcanic Complex
The following study is by Breeman et al. (2001): The Black River Volcanic Complex is an Archaean (2.708 to ca. 2.66 Ga) stratovolcano that lies 480 km. northeast of Yellowknife, Northwest Territories. It constitutes the Back Group of the Yellowknife Supergroup and is somewhat anomalous in the Slave craton because it has undergone only a low degree of deformation and is subhorizontal. The southern half of the complex is exposed at the crest of a small dome. This is the eroded portion of the stratovolcano that has been preserved in an upright position. The complex comprises four volcanic sedimentary sequences (Innerring, Thlewyco, Boucher-Regan, Kelsh) that correspond to the phases of growth and destruction of this stratovolcano.
The Innerring sequence, which constitutes the oldest rocks of the complex, represents the upper part of an eroded early phase of the volcano (U-Pb zircon igneous age of 2.708 Ga). The Thlewyco sequence represents the main construction phase of the volcano and forms an outward dipping, annular succession around the Innerring sequence, with an aggregate thickness of 2500-5000 meters. Its stratigraphy changes dramatically around the crator, varying from five cycles of andesitic and rhylotic lava, followed by succession of volcanistic debris on the north side; to 30 subarial andesitic flowqs and rare pyroclastic and epivolcaniclastic units on the eastern side; to interlayered dacitic and andesitic lava and tuff overlain by a thick succession of voluminous, nonwelded, ash-flow tuff and volcaniclastic rocks on the south side. Volcanism in this sequence ended with the eruption of large rhyolite and dacite dome-flow complexes (U-Pb zircon dated at 2.692 Ga). The Innerring and Thlewycho sequences represent a complex history of explosive erruptions from numerous eruptive centers.
The Boucher-Regan sequence, with its predominance of pillowed lava flows, suggests that this northern-flank of the volcano was submerged during deposition. The Kelsh sequence on the northwestern side comprises epiclastic volcarenite, rhyolite-dacite block breccia from lava domes, polymict breccia, and a conglomerate of andesite, dacite-rhyolite clasts, and andesitic tuff. The exposed succession consists of iron-formation, oolitic-stromatolitic carbonate, sulphidic volcaniclastic rocks and graphitic slate that marks the end of volcanism. The Kelsh sequence forms a broad apron that is interpreted as a shallow, sunmarine to subaerial, clastic fan derived by degradation of the volcanic pile (U-Pb zircon dated at 2.586 Ga). (Breeman et al., 2001).
Age determinations in the Black River complex are similar to the ages of volcanic rock found in the upper Kam Group of the Yellowknife greenstone belt as well as other volcanic centers in the western Slave province. Detrital zircon geochronology on turbiditic sequences confirm a secondary deposition of turbidite units at ca. 2.62-2.60 Ga that was widespread throughout the Slave province. (Breeman et al., 2001).
[edit] Southern Slave province, Yellowknife greenstone belt
The Yellowknife greenstone belts consists of crossbedded orthoquartzite, together with >3.0 Ga gneissic xenoliths that cut through the belt. U/Pb geochronologic data of the greenstone belt and the adjacent granitoid rocks confirm that some of the granitoid rocks below the greenstone belt represent old basement. Plutonic activity occurred in the underlying basement at ca. 2.95 Ga. Detrital zircons in the quartzite north of Yellowknife have been dated at >3.9 Ga. (Bleeker et al., 2000). Seafloor hydrothermal deposits and alteration and minor sulphide mineralization occurs in the Yellowknife belt (e.g., the Homer Lake showing, and horizons northeast of Bell Lake). Neoarchean bimodal rift volcanic rocks overlie faulted basement in the basal greenstones of the Yellowknife belt.
The late Archean stratigraphy in Yellowknife is compatible with the rifting interpretation outlined above in the "Post-2.70 Ga volcanism" section of this article. In Yellowknife, the top of the sequence is represented by voluminous basaltic flows and intercalated felsic volcanic rocks of the Yellowknife Bay Formation, dated at ca. 2.7 Ga. In support of the overall regional correlation, similar ca. 2.7 Ga ages have been obtained from Courageous Lake and Acasta areas. Stratigraphy, dense dyke swarms, and isotopic data link the basalt sequence to the basement. At the top of the Kam Group, bimodal volcanic rocks of the Yellowknife Bay Formation become progressively more intercalated with volcaniclastic sediments, before final intrusion by thick tholeiitic sills. One of these sills, the Kam Point gabbro sill, has a preliminary baddeleyite age of ca. 2.697 Ga. (Bleeker, 2006).
For the Burwash Formation of the Yellowknife Domain refer to the above "Ca. 2.68-2.66 Sedimentation" section of this article.
[edit] Economic geology
Proterozoic rift-related magmatic suites and arcs around the margins of the craton host a variety of mineral deposits, and several hundred Phanerozoic kimberlites support Canada's first diamond mines. Geologic interest has been renewed in the craton by the discoveries of kimberlite diamond occurrences in the region. The Eocene (ca. 55-50 Ma) kimberlite pipes of the Lac de Gras area in the central Slave craton now support two highly profitable diamond mines, Ekati and Diavik. Several other diamond mines are in various stages of development. In just over a decade, diamonds have become the most profitable commodity within this ancient craton.
The banded iron formations (BIFs) of the Slave craton appear to be of low economic value, although some may possibly host epigenetic gold mineralization and may be underexplored. However, most iron formation-hosted gold mineralization appears to be associated with BIFs hosted in low to medium-grade turbidite packages. However, the Burwash Formation metaturbidites and the Yellowknife greenstone belt do have epigenetic gold mineralization. Epigenetic gold mineralization throughout the Slave craton were caused by deformation and associated metamorphism. In the Yellowknife greenstone belt, it led to formation of the ca. 15 million ounce Con-Giant Au deposit, along a complex system of mostly reverse shear zones. As is typical for this class of deposits, the Con-Giant system occurs mostly within moderate to strongly deformed basaltic rocks, in proximity to a regional stratigraphic break, the Yellowknife River Fault Zone. An asymmetric synclinal panel of syn-orogenic conglomerates (the Jackson Lake Formation) occurs along this fault zone. Identical relationships are observed in several other major Archean gold camps, most notably Timmins, Kirkland Lake, and Kalgoorlie. The critical control common to all these camps is localization of Au mineralization within significant bends of the regional fault zones; these bends were most likely dilational during emplacement of the gold-bearing quartz veins.
Although numerous other volcanic-hosted gold vein systems are known from the Slave craton, some of which were briefly in production in the past, only one other major camp has emerged in recent years. This camp occurs in the Hope Bay belt, on the Coronation Gulf coast of the Slave craton, and consists of a string of deposits (Boston, Doris, Madrid) that are being readied for production. Elsewhere, despite significant past exploration, overall potential for this class of deposits remains excellent. Several greenstone belts throughout the Slave craton have very similar structural-stratigraphic characteristics to that of the Yellowknife belt, including a thick, folded turbidite pile adjacent to a basaltic greenstone belt, a regional deformation zone, and a young conglomerate package. Best examples are the Point Lake and Arcadia Bay areas. (Bleeker, 2006).
Another class of mineral deposits related to final orogenesis is that of rare-element enriched granitoids, particularly highly evolved anatectic granites and their pegmatites. Tin (cassiterite) and Li (spodumene) were briefly mined from such pegmatites in the Yellowknife Domain, but other pegmatite fields are known throughout the Slave craton. From a global metallogeny point of view, these occurrences are of interest as they typically correlate with ancient, multiply recycled, felsic crust. (Bleeker, 2006).
Fuchsitic quartzites below the iron formations are enriched in detrital minerals, including highly stable heavy minerals such as chromite, zircon, and rutile. Individual, dark, detrital chromite grains are a characteristic feature of these otherwise white to grey quartzites. Commonly, these chromite grains have undergone variable reaction towards bright green fuchsitic mica during metamorphism and deformation. In a few localities, chromites are concentrated in seams of "black sand", but clearly such concentrations are too small to be of economic interest. If road access were available, some of the green-white quartzite would make attractive building or decorative stone. In Greenland, India, and Australia, similar quartzites are often quarried for this purpose. Elsewhere in the world, quartzites similar to these contain paleoplacer deposits of gold and/or uranium. (Bleeker, 2006).
[edit] See also
[edit] References
- Bleeker, Wouter. (2006) ""Mineral Resources of Canada: A Synthesis of Major Deposit-types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods." paper to be published jointly by the Geological Survey of Canada (GSC) and the Mineral Deposits Division (MDD) of the Geological Association of Canada. Current online verion: [1]
- Bleeker, Wouter, Bill davis, John Ketchum, Richard Stern, Keith Sircombe, and John Waldron. (2004) "The Slave Craton From On Top: The Crustal View." Geological Survey of Canada.[2]
- Bleeker, Wouter, Bill Davis, Herman gritter, and Alan G. Jones. (2004a) "The Slave Craton From Underneath: The Mantle View." Continental Geoscience Division, Geological Survey of Canada. [3]
- Bleeker, Wouter, K. Sircombe, and R. Stern. (2000) "Why the Slave Province, Northwest Territories, got a little bigger." Geological Survey of Canada Bookstore. Online: [4]
- Geologic map
- Bowring, S.A., and Williams, I.S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, v. 134, 3-16.
- Breeman, Otto Van, Maurice Lambert, Jim Mortensen, and Mike Vileneuve. (2001) "Geochronology of the Black River Volcanic Complex, Nunavut-Northwest Territories." Geological Survey of Canada, Current Research 2001-F2:[5]
- Stern, R.A., Bleeker, W., 1998. Age of the world's oldest rocks refined using Canada's SHRIMP. the Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada, v. 25, p. 27-31
