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Reed Lake Geology Trans-Hudson Orogen Overview The Paleoproterozoic Trans-Hudson Orogen (THO; Fig. 2; Hoffman, 1989; Lewry and Stauffer, 1990) extends from South Dakota, through the exposed Shield in Saskatchewan and Manitoba, across Hudson Bay to northern Quebec. The orogen is part of a greater "Pan-American" Paleo- to Mesoproterozoic system whose evolution involved assembly of dispersed Archean minicontinents and accreted juvenile Paleoproterozoic terranes during the main episode of North American continental assembly (Hoffman, 1989). Time-space relations of lithotectonic elements in the Trans-Hudson Orogen and the "Pan-American" system are similar to those in younger orogens formed by subduction/accretion/collision at convergent plate boundaries. In the Saskatchewan-Manitoba segment, four major lithotectonic zones are recognized: * Superior Boundary Zone, a narrow, southeastern, ensialic foreland zone bordering Superior craton, comprising the Thompson Belt, Split Lake Block, and Fox River Belt * The internal Reindeer Zone, a 400 km wide collage of Paleoproterozoic (1.92-1.83 Ga) arc volcanic rocks, plutons, volcanogenic sediments, and younger molasse, divisible into several lithostructural domains. Geochemical and Nd and Pb isotopic data indicate that most of these rocks evolved in an oceanic to transitional, subduction-related arc setting, with increasing influence of Archean crustal components to the northwest. The Reed Lake-Snow Lake Domain, for example, is interpreted as an imbricated thrust wedge carried on a lower detachment zone and overridden by higher grade Kisseynew gneisses (Lewry et al., 1990; Lucas et al., 1994, 1997). The Reindeer Zone overlies Archean basement exposed in structural windows (Lewry et al., 1990); this basement terrane is now termed the "Sask craton" (Ansdell et. al., 1995). * An Andean-type continental-margin, magmatic arc, represented by the Wathaman-Chipewyan Batholith emplaced at 1.86-1.85 Ga (Meyer et al., 1992). * A complexly deformed Northwestern Hinterland Zone, including the Peter Lake, Wollaston, and Seal River domains, and other parts of the Cree Lake Zone now included in Hearne Province (Hoffman, 1989; Lewry and Stauffer, 1990). From: Syme, E.C., Lucas, S.B., Zwanzig, H.V., Bailes, A.H., Ashton, K.E., and Haidl, F.M., 1998: Geology, NATMAP Shield Margin Project Area Reed Lake Belt, Manitoba/Saskatchewan accompanying notes, Geological Survey of Canada, Maps 1968A, A-98-2, 258A.  The Reed Lake Belt The Reed Lake Belt is in the juvenile internal zone of the Trans-Hudson Orogen and consists of Paleoproterozoic volcanic, plutonic and minor sedimentary rocks. The exposed portion of the belt is 250 km long by 75 km wide. Although it has an apparent easterly trend, this is an artifact of the belt's tectonic contact with gneissic metasedimentary, metavolcanic and plutonic rocks to the north (Kisseynew Domain) and the east-trending trace of Phanerozoic platformal cover rocks to the south. In reality the Reed Lake greenstone belt extends hundreds of kilometres to the south-southwest beneath a thin, geophysically transparent Phanerozoic cover. To the north the Reed Lake Belt is tectonically overthrust by younger metasedimentary rocks of the Kisseynew domain and by nappes of metavolcanic rocks that are the same age as those in the Reed Lake Belt. The NATMAP Shield Margin Project and LITHOPROBE Trans-Hudson Orogen Transect have shown that, on the scale of the crust, the Reed Lake greenstone belt (and contained VMS deposits) is only one of three components in a northeast-dipping stack, juxtaposed during 1.84-1.80 Ga collisional deformation * at the lowest structural level (exposed in the Pelican Window): metaplutonic rocks and paragneisses (3.20-2.40 Ga) of the "Sask craton". * at intermediate structural levels: Reed Lake Belt (now defined to include the Attitti Block and Paleoproterozoic rocks in the Hanson Lake Block) and Glennie Domain (together comprising the "Reed Lake-Glennie Complex"; Lucas et al. 1997). * at the highest structural levels: marine turbidites (Burntwood Group; 1.85-1.84 Ga) and partly coeval distal facies of alluvial-fluvial sandstones (Missi Group) in the Kisseynew Domain. The Reed Lake Belt is a typical greenstone terrain and was once interpreted to be Archean in age (Harrison, 1951; Stockwell, 1961) based on its lithological, structural, and metamorphic similarities with greenstone belts in the Superior Province. Historically, the stratigraphy of the Reed Lake Belt has been described in terms of two stratigraphic groups, Amisk Group volcanic rocks and Missi Group continental sedimentary rocks (Bruce, 1918; Harrison, 1951). The Reed Lake Belt, and in particular the Amisk Group, is now recognized to be a collage of distinct tectonostratigraphic assemblages that was assembled prior to the emplacement of voluminous granitoid plutons and regional deformation related to the Ca. 1.8 Ga Hudsonian Orogeny. This is the basis for Lucas et al. (1996) terming the tectonic entity between the Sturgeon-weir River and Reed Lake as the "Amisk collage",and for rejecting Amisk Group as the term to describe the 1.92-1.87 Ga volcano-plutonic rocks. The Reed Lake Belt is composed of structurally juxtaposed volcanic and sedimentary assemblages that were emplaced in a variety of tectonic environments. The major 1.92-1.88 Ga components include a very significant juvenile arc and juvenile ocean-floor rocks, and minor ocean plateau/ocean island basalt. The juvenile arc assemblage comprises tholeiitic, calc-alkaline and lesser shoshonitic and boninitic rocks similar in major and trace element geochemistry to modern intraoceanic arcs. Ocean-floor basalt sequences are exclusively tholeiitic, and are geochemically similar to modern N- and E-type MORBs erupted in back-arc basins. Evolved arc assemblages and Archean crustal slices are present within the Reed Lake Belt as minor components. Collectively, these tectonostratigraphic assemblages were juxtaposed in an accretionary complex at Ca. 1.88-1.87 Ga, presumably as a result of arc-arc collisions. The collage was basement to 1.87-1.83 Ga post-accretion arc magmatism, expressed as voluminous caic-alkaline plutons and rarely preserved caic-alkaline to alkaline volcanic rocks. Unroofing of the accretionary collage and deposition of continental alluvial-fluvial sedimentary rocks (Missi Group) and marine turbidites (Burntwood Group) occurred Ca. 1.85-1.84 Ga, coeval with the waning stages of post-accretion arc magmatism. The sedimentary suites were imbricated with volcanic assemblages in the eastern Reed Lake Belt during 1.85-1.82 Ga juxtaposition of the supracrustal rocks along pre-peak metamorphic structures. Post ca. 1.83 Ga structures formed the present southwest-verging fold style at the northeastern end of the Reed Lake Belt. The tectonostratigraphic architecture of the Reed Lake Belt is of essential economic significance. The belt is one of the largest Proterozoic volcanic-hosted massive sulphide (VMS) districts in the world, containing 27 Cu-Zn- (Au) deposits from which more than 183 million tonnes of suiphide have already been mined. Most of mined VMS deposits in the Reed Lake belt are associated with the juvenile arc volcanic rocks, providing a powerful focus for exploration in the belt. New geochronological work indicates, however, that at least one of the mined deposits is hosted by rhyolites dated at 1869 Ma and thus is associated with the post-accretion arc magmatism. Gold mineralization in the Reed Lake Belt is less thoroughly studied but at Reed Lake has been shown to be intimately associated with late brittle-ductile shear zones that follow peak tectonic and metamorphic activity within the Trans-Hudson Orogen. At Snow Lake, however, preliminary investigations suggest a long history of gold mineralization with at least some gold introduced prior to metamorphism. (Alan Bailes and Ric Syme, 2001) |
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