Cat Lake–Euclid Lake area in the Neoarchean Bird River greenstone belt, southeastern Manitoba (parts of NTS 52L11, 12): preliminary results of bedrock geological mapping and their implications for geodynamic evolution and metallogeny

This report presents the preliminary results of bedrock geological mapping conducted at a scale of 1:10 000 in the Cat Lake–Euclid Lake area by the Manitoba Geological Survey in the summer of 2013. The map area is located in the northern arm of the Bird River greenstone belt2, southeastern Manitoba, about 145 km northeast of Winnipeg. The main part of this area is underlain by a Neoarchean supracrustal assemblage comprising volcano-sedimentary and intrusive rocks, which is bounded to the south by the Mesoarchean Maskwa Lake batholith and to the north by sedimentary3 rocks and derived gneissic, weakly foliated to massive granitoid rocks. The supracrustal assemblage consists of 1) mafic volcanic and synvolcanic intrusive rocks, 2) epiclastic and minor volcaniclastic rocks, and 3) mafic–ultramafic intrusions. Both the Mesoarchean granitoid batholith and supracrustal rocks are intruded and disrupted by a Neoarchean tonalite-trondhjemite-granodiorite (TTG) suite and late peraluminous granitoid rocks and associated pegmatites. The mafic–ultramafic intrusions are emplaced within a mid-ocean-ridge basalt (MORB)–type sequence that is extensive in the northwestern part of the map area, and are in fault contact with granitoid and sedimentary rocks in the southeastern part. Some of the mafic–ultramafic intrusions are associated with platinum-group element (PGE)–Ni–Cu–Cr mineralization, and thus have become targets for mineral exploration. Rare-metal (Li, Cs, Nb, Ta) mineralization is confined to pegmatite intrusions associated with peraluminous granitoid rocks that are relatively younger than the TTG suite, in which pegmatite and aplite intrusions are, in contrast, compositionally simple and devoid of metasomatic textures. There exists, however, a potential for porphyry Cu-(Au) mineralization associated with some granitoid phases in the Neoarchean TTG suite. The results of new and previous mapping suggest that the MORB-type basalts and related synvolcanic intrusive rocks, as well as the mafic–ultramafic intrusions, may have been emplaced in an extensional setting at a continental margin, possibly represented by older granitoid rocks in the Maskwa Lake batholith (i.e., Maskwa Lake batholith I, ca. 2853–2782 Ma; Gilbert et al., 2008). The Neoarchean mafic–ultramafic intrusions, including the Cat Lake, New Manitoba Mine and Euclid Lake (U-Pb zircon age of 2743 Ma) intrusions, consist of a diversity of rock types, including gabbro, leucogabbro to anorthositic gabbro, melagabbro, amphibolite and/or pyroxenite, and peridotite. Whereas all three of the these intrusions may be coeval and thus the products of the ‘Bird River magmatic event’ (Houlé et al., 2013), the precise age and affinity of the Cat Lake and New Manitoba Mine intrusions are yet to be determined. The TTG suite, which includes Neoarchean phases of the Maskwa Lake batholith (Maskwa Lake batholith II, 2725 ±6 Ma; Wang, 1993), as well as the Inconnu pluton I (Černý et al., 1981), may have been formed in a magmatic-arc setting; subsequent emplacement of peraluminous granitoid rocks and associated rare-metal-bearing pegmatites may have occurred during continental collision subsequent to plate subduction. The north-northwest-trending Cat Lake–Euclid Lake dextral shear zone is confined to gneissic, peraluminous granitoid rocks, as well as strongly foliated and mylonitic granitoid rocks that may mark the southern boundary of the English River subprovince.