GS-9 Geological investigations in the Manasan Falls area, Thompson Nickel Belt, Manitoba (part of NTS 63P12) by C.G. Couëslan Couëslan, C.G. 2011: Geological investigations in the Manasan Falls area, Thompson Nickel Belt, Manitoba (part of NTS 63P12); in Report of Activities 2011, Manitoba Innovation, Energy and Mines, Manitoba Geological Survey, p. 86–93. Summary Superior craton collided with the Mapping in the Manasan Falls area, located near adjacent arc-derived Kisseynew the western margin of the Thompson Nickel Belt (TNB) Domain of the Reindeer Zone. southwest of Thompson, has revealed the presence Initial collision (ca. 1.83– of a supracrustal sequence that is likely Archean. The 1.80 Ga) resulted in large-scale isoclinal to recumbent F2 sequence consists of banded para-amphibolite or mafi c folds and the development of the regionally penetrative metavolcaniclastic rock, siliceous metasedimentary rock, S2 foliation (Ansdell, 2005; Burnham et al., 2009). metagreywacke, and metapelite and metasemipelite. Later transpressional movement (D3, ca. 1.77–1.76 Ga) Other units in the area consist of Archean multicomponent resulted in elongate chains of upright, doubly-plunging gneiss, white quartzite of unknown affi nity and an augen F3 synforms and antiforms (Bleeker, 1990; Burnham et gneiss–granite gneiss suite, which is interpreted as al., 2009). Mylonite zones with subvertical-stretching Paleoproterozoic. The new fi ndings at Manasan Falls, lineations parallel many of the F3 fold limbs (Bleeker, together with recent discoveries of Archean supracrustal 1990). rocks elsewhere in the TNB, suggest that some occurrences The Kisseynew Domain dominantly consists of supracrustal rocks have been misinterpreted as rocks of Burntwood Group, a sequence of deeper-water of the Paleoproterozoic Grass River or Ospwagan metagreywacke and metamudstone (Zwanzig et al., 2007). groups, and that Archean supracrustal rocks may be more The Grass River Group, a shallow-water equivalent to the widespread than previously recognized. Burntwood Group, occurs along the eastern margin of the Kisseynew Domain. The Grass River Group consists of Introduction polymictic metaconglomerate and layered felsic to mafi c In July 2011, the water levels on the Burntwood metasandstone (Zwanzig, 1998; Zwanzig et al., 2007). River in Thompson were 4–5 feet below average levels The adjacent, eastern Kisseynew Domain has a shared (Manitoba Hydro, 2011). The low water levels allowed tectonic history with the TNB, beginning with initial for the examination of extensive outcrops in the Manasan collision at ca. 1.83 Ga. Falls area that are typically submerged or otherwise The Manasan Falls area is located along the western inaccessible. Previous work from diamond drillcore led to edge of the TNB, near the fault-bounded contact with metamorphosed supracrustal rocks, tentatively identifi ed the adjacent Kisseynew Domain (Figure GS-9-1). The as Grass River Group, being recognized in the Manasan area is underlain by Archean gneiss and what has been Falls area (Macek et al., 2006; Zwanzig, pers. comm., tentatively identifi ed from diamond drillcore as Grass 2011). Mapping of the area was conducted over several River Group metasedimentary rocks (Macek et al., 2006; days at the end of July in an attempt to positively identify Zwanzig, pers. comm., 2011). The falls are situated on the these rocks. eastern limb of an upright F3 antiform. Geological overview Lithological units The TNB forms part of the northwestern Superior Five main lithological units were identifi ed in the Boundary Zone of Manitoba. It is predominantly underlain Manasan Falls area (Figure GS-9-2). The majority of by Archean gneiss of the Superior craton. The Archean rocks are interpreted as Archean, the exceptions being gneiss is described as mostly felsic to intermediate the quartzite, which is of unknown affi nity, and the augen orthogneiss (Bleeker, 1990); however, work along the gneiss, which may be Paleoproterozoic. The relative eastern side of the TNB has revealed the presence of belts of ages of individual units are not known due to the lack of Archean metasedimentary rocks (Böhm, 2005; Couëslan, exposed contacts and preserved way-up indicators in the 2008, 2009). Prior to ca. 1.88 Ga, the Paleoproterozoic metasedimentary rocks. The order in which the units are passive-margin sequence of the Ospwagan Group was discussed here does not follow either their chronological deposited on exhumed Archean gneiss of the Pikwitonei or their stratigraphic order. Although all rocks have Granulite Domain (Bleeker, 1990; Zwanzig et al., 2007). been subjected to at least upper-amphibolite–facies These rocks were deformed and metamorphosed during metamorphic conditions, the “meta-” prefi x has been the Trans-Hudson orogeny (ca. 1.83–1.76 Ga), when the omitted in the following to improve readability. 86 Manitoba Geological Survey 6 212 000N 596 800E ? Moak Lake e ak L y r e t s y M Burntwood River Figure GS-9-2 Thompson r e iv R d o r Birchtree o iv e w t R n r n u a Legend s B a n Granitoid rocks a M Kisseynew Domain Grass River and Sickle groups Burntwood Group Thompson Nickel Belt Ospwagan Group Reworked Archean gneiss Ospwagan Lake Superior Province Pikwitonei granulite Mine site City / town Pipe Paint Lake 10 km 548 700E 6 147 400N Figure GS-9-1 : Geology of the northern Thompson Nickel Belt and location of the study area at Manasan Falls, Manitoba (modifi ed from Macek et al., 2006). Report of Activities 2011 87 6 177 500N 566 000E 5a 75 5b 82 3 78 ng ppi 1 ma t of ten Ex iver 80 d R oo tw rn 2 u 80 B 4 1 72 3 1 85 5b Legend 84 75 Control 5 Granitoid gneiss structure a: augen gneiss b: granite gneiss 77 1 Manasan 4 Quartzite ng Falls pi ap 3 Layered mafic sequence 5a f m t o 2 Greywacke 88 en xt 1 E Multicomponent gneiss 74 3 Mylonite zone M Foliation (vertical, known) ana s Minor shear zone (known) a n 1 R 0 500 1000 569 000E i v 2 e r metres 6 174 000N Figure GS-9-2: Geology of the Manasan Falls area, Manitoba. Multicomponent gneiss light grey to pinkish grey, medium- to coarse-grained and Outcrops of multicomponent gneiss, the dominant migmatitic (Figure GS-9-3a); its composition varies from rock type in the Manasan Falls area (Figure GS-9- tonalitic to granodioritic, with 10–20% biotite and/or 2), consist of either biotite- or hornblende-gneiss and hornblende. Amphibolite occurs as disaggregated bands typically include trace to 3% ultramafi c schist, trace to up to 3 m thick that are commonly boudinaged. Local 3% calcsilicate, 3–25% pegmatite and 7–20% plagioclase boudins of calcsilicate (3–30 cm thick) and ultramafi c amphibolite. Rare lenses of iron formation, psammite and schist (up to 1 m thick) occur along discrete horizons. clinopyroxenite also occur. Multicomponent gneiss is Pegmatite dikes are generally subparallel to the foliation, 88 Manitoba Geological Survey a b c d e f Figure GS-9-3: Outcr op photographs in the Manasan Falls area, Manitoba: a) multicomponent gneiss with discontinuous bands of plagioclase amphibolite; b) mylonitized multicomponent gneiss located just east of Manasan Falls, Manitoba; c) greywacke with veins of leucosome that are subparallel to bedding; d) white quartzite of unknown affi nity located upstream of Manasan Falls; e) a typical example of the augen gneiss; f) weakly foliated porphyritic granodiorite. Scale card is in centimetres. but also occur as irregular intrusions, and are variably decimetre-scale) and iron formation (up to 50 cm), foliated. Outcrops of multicomponent gneiss located discontinuous bands of plagioclase amphibolite (up to directly east of the falls are mylonitic (Figure GS-9-3b) 1 m) and rare boudinaged layers of calcsilicate (up to and locally intensely folded. 1 m). The greywacke forms a light grey, medium-grained paragneiss with centimetre- to decimetre-scale bands. It is typically quartz-rich (40–50%) and contains plagioclase Greywacke (20–30%), biotite (10–30%), garnet (5–15%) and, rarely, Greywacke is most common along the eastern side sillimanite. Outcrops typically contain millimetre- to of the Burntwood River (Figure GS-9-2). It locally centimetre-scale bedding-parallel veins of leucosome, contains intercalations of psammite (centimetre- to which are characterized by pinch-and-swell structures Report of Activities 2011 89 (Figure GS-9-3c). Directly south of the falls, a contact of banded amphibolite, which grades into siliceous between the multicomponent gneiss and greywacke is sedimentary rock and pelite towards the northwest, and visible in outcrop; the contact consists of intercalations which is interlayered with greywacke, semipelite and of greywacke and multicomponent gneiss within a zone calcareous rocks towards the southeast. All phases within roughly 2 m wide. It is not clear if this interleaving is the the layered mafi c sequence appear migmatitic. Veins of result of tectonic or sedimentary processes. A sedimentary pseudotachylyte occur locally in central portions of the origin would imply that the multicomponent gneiss at outcrop. some locations is derived from an arkosic rock. Banded amphibolite Quartzite Banded amphibolite is a compositionally variable, A small outcrop of white quartzite, located in a layered sequence roughly 40 m thick; it is dark grey to quarry upstream of the falls, is in sharp contact with the greenish grey and banded on a centimetre- to decimetre- multicomponent gneiss (Figure GS-9-3d). The quartzite scale (Figure GS-9-4a). The rock is hornblende-rich with is interbedded with local protoquartzite horizons up to variable proportions of plagioclase, biotite and up to 30% 40 cm thick and contains bands of plagioclase amphibolite quartz. Local calcsilicate bands (2–20 cm) contain green up to 20 cm wide. The quartzite is medium grained and, amphibole and/or diopside in place of hornblende and in addition to quartz, contains minor plagioclase (7–10%), biotite (Figure GS-9-4b).