GS-8 Tectonometamorphic investigations in the Athapapuskow Lake area, west-central Manitoba (part of NTS 63K12) by M. Lazzarotto1, S. Gagné and D.R.M. Pattison1 Lazzarotto, M., Gagné, S. and Pattison, D.R.M. 2016: Tectonometamorphic investigations in the Athapapuskow Lake area, west-central Manitoba (part of NTS 63K12); in Report of Activities 2016, Manitoba Growth, Enterprise and Trade, Manitoba Geological Survey, p. 87–98. Summary metamorphic reactions transport The Athapapuskow Lake area is part of a tectonic col- and concentrate precious met- lage situated in the Flin Flon greenstone belt, Manitoba. It als. Identifying these transitions consists of accreted terranes comprising metamorphosed therefore has implications for gold exploration. ocean-floor and island-arc assemblages unconformably The objective of the project is to refine the metamor- overlain by sedimentary rocks and intruded by successor- phic and tectonic history of the Athapapuskow Lake area arc plutons. The area consists of blocks bound by faults established by previous workers (e.g., Syme and Bailes, and major shear zones, within which both regional and 1989; Syme, 2015) based on field mapping and petrog- contact metamorphism are observed. The regional meta- raphy. morphic grade generally increases northward across the The Athapapuskow Lake area was selected for sev- Flin Flon belt from prehnite-pumpellyite–facies rocks in eral reasons: 1) the area straddles the transition from the south through to amphibolite-facies rocks in the north, prehnite-pumpellyite, through greenschist, to amphibo- but this general pattern is locally disrupted by faults and lite facies; 2) several different shear zones and faults plutons. Rocks in contact metamorphic aureoles around of regional tectonic relevance cut the area, juxtaposing plutons record proximal amphibolite-facies conditions. blocks of sometimes considerably different origin and Late shear zones overprint the margins of the plutons and metamorphic grade; and 3) relationships between meta- the contact aureoles. A preliminary metamorphic map of morphism in contact aureoles and the later regional meta- the area is presented, together with new isograd maps for morphic overprint can be studied around several plutons. the contact aureole of the Lynx Lake pluton and for fault- Results of these investigations will allow to test the tradi- bounded blocks in the Schist Lake region. Displacement tional model for the tectonic evolution of the western Flin of isograds along faults, kinematic indicators and rela- Flon belt (i.e., early intra-oceanic accretion, followed by tive timing of movement on the shear zones indicate that successor arc plutonism, followed by regional thermotec- vertical and strike-slip movements played an important tonism; Lucas et al., 1996), with implications for base- role in the tectonometamorphic evolution of the region. Relationships and timing of metamorphism and structures and precious-metals exploration. suggest that peak metamorphic conditions may have been During two field seasons (summer 2015 and 2016), attained during an early stage in the tectonometamorphic targeted mapping and sampling was completed in the evolution of the Schist Lake area. Athapapuskow Lake area and in the nearby North Star Lake and File Lake areas. This report focuses on results from the Athapapuskow Lake area. A preliminary new Introduction metamorphic map of the Athapapuskow Lake area, includ- In 2015, the University of Calgary, in collaboration ing newly defined isograds for an area around Schist Lake with the Manitoba Geological Survey, started a new project and the contact aureole of the Lynx Lake pluton, is pre- involving the tectonic and metamorphic study of parts sented. Relationships and timing of metamorphism and of the Flin Flon–Snow Lake greenstone belt. In addition structures are discussed. Future research will focus on the to providing insight into tectonic and metamorphic pro- investigation of relationships along a north–south transect cesses, such as behaviour of rocks at the transition from in the File and North Star lakes areas. low-grade (prehnite-pumpellyite facies) to medium high- grade (greenschist- to amphibolite-facies) metamorphism, an improved understanding of the tectonometamorphic Regional geology evolution of this area provides information to constrain The Athapapuskow Lake area is situated in the west- exploration models for volcanogenic massive sulphide ern Flin Flon belt, Manitoba (part of NTS 63K12) and is (VMS) and orogenic gold deposits in the region. For part of the juvenile Reindeer zone of the Trans-Hudson example, orogenic gold deposits are often situated in brit- Orogen (Figure GS-8-1; Hoffmann, 1988). Convergence tle-ductile shear zones near transitions from greenschist- between the Hearne and Superior cratons resulted in a geo- to amphibolite-facies rocks, where fluids released during logical setting characterized mainly by the juxtaposition 1 Department of Geosciences, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4 Report of Activities 2016 87 Figure GS-8-1: Generalized geology of the Flin Flon–Snow Lake greenstone belt (modified from NATMAP Shield Margin Project Working Group, 1998). The box outlined in red indicates the Athapapuskow Lake area (Figure GS-8-2). of juvenile-arc and juvenile–ocean-floor rocks, accompa- patterns of faults and folds (Figure GS-8-2a; Bailes and nied by contaminated-arc, ocean-island and ocean-plateau Syme, 1989; Stern et al, 1995a; Lucas et al, 1996). The rocks (Stern et al., 1995a, b; Syme et al., 1999; Whalen et arc-related assemblages comprise a wide range of vol- al., 2016). canic, volcaniclastic and related synvolcanic intrusive rocks, whereas the ocean-floor assemblages are mainly Circa 1920–1880 Ma juvenile-arc and ocean-floor composed of mid-ocean-ridge–like basalt and related rocks, including tholeiitic, calcalkalic and lesser shoshon- mafic–ultramafic complexes (Syme and Bailes, 1993; itic and boninitic rocks, were juxtaposed in an accretion- Stern et al., 1995b; Lucas et al., 1996). Sedimentary rocks ary collage, probably as a consequence of arc-arc collision principally consist of thick sequences of continentally at about 1880–1870 Ma (Lucas et al., 1996). This became derived conglomerate and sandstone. the starting point for postaccretion and successor-arc magmatism, which resulted in the emplacement of cal- The rocks show a metamorphic overprint ranging calkalic plutons ca. 1870–1830 Ma (Lucas et al., 1996). from sub-greenschist facies to amphibolite facies (Fig- Between 1850 and 1840 Ma, continental sediments of the ure GS-8-2b; Digel and Gordon, 1995; Syme, 2015). A Missi group were deposited (Ansdell et al., 1995; Lucas northward increase in metamorphic grade within single et al., 1996). The complex imbrication of sedimentary blocks is observed. In general, the grade changes north- and volcanic assemblages along pre-peak metamorphic ward from prehnite-pumpellyite, through greenschist, to structures (Lucas et al., 1996; Stern et al., 1999) is the amphibolite facies starting at the unconformable contact consequence of two additional collisional events, which with overlying Paleozoic sediments in the southern part resulted in what is known as the ‘Amisk collage’ (Lucas of Athapapuskow Lake (Bailes, 1979; Digel and Gordon, et al., 1996). The first event involved the Sask craton at 1995). 1840–1830 Ma and the second, immediately afterwards, involved the Superior craton at 1830–1800 Ma (Bleeker, Structures 1990; Ellis et al., 1999; Ashton et al., 2005). Faults and shear zones that separate distinct strati- Today the Athapapuskow Lake area of the Flin Flon graphic packages in the Athapapuskow Lake area were belt consists of imbricated juvenile-arc, ocean-floor and termed ‘block-bounding structures’ by Bailes and Syme sedimentary rocks, characterized by internally complex (1989). These structures vary from abrupt faults (e.g., 88 Manitoba Geological Survey Figure GS-8-2: a) Generalized geology of the Athapapuskow Lake area (modified from Syme, 2015). Boxes outlined in red indicate locations of Figures GS-8-4 and GS-8-6. b) Preliminary map of metamorphic facies in the Athapapuskow Lake area. Abbreviations: NEASZ, Northeast Arm shear zone; pl., pluton. Report of Activities 2016 89 Ross Lake fault, labelled ‘c’ in Figure GS-8-2b) to wide be affected by the movements along the shear zone, sug- zones of intensely sheared rocks (e.g., Northeast Arm gesting that the displacement along the fault ceased once shear zone, labelled ‘a’ in Figure GS-8-2b). Some of these the folding took place. These observations do not exclude structures were re-investigated in the course of this proj- the possibility of some degree of younger reactivation in ect and are briefly described here. other parts of the shear zone. Northeast Arm shear zone Inlet Arm shear zone The Northeast Arm shear zone (Figure GS-8-2b) is a north-northeast-trending structure characterized by a zone The Inlet Arm shear zone (Figure GS-8-2b) consists of mylonite and tectonite up to 1000 m wide. In the study of a main discrete fault surrounded by a broad (800 m) area, it juxtaposes substantially different assemblages, zone of ductile deformation. The shear zone trends north namely mafic extrusive rocks of the Bear Lake block to and separates mainly mafic volcaniclastic rocks of the the west with intrusive rocks of the Airport and Schist Hook Lake block to the west from mafic extrusive rocks Creek plutons to the east (Figure GS-8-2a). of the Bear Lake block to the east.