GS-5 Preliminary results of bedrock mapping at Bigstone Lake, northwestern Superior province, Manitoba (parts of NTS 53E12, 13) by M.L. Rinne, S.D. Anderson and K.D. Reid Rinne, M.L., Anderson, S.D. and Reid, K.D. 2016: Preliminary results of bedrock mapping at Bigstone Lake, northwest- ern Superior province, Manitoba (parts of NTS 53E12, 13); in Report of Activities 2016, Manitoba Growth, Enterprise and Trade, Manitoba Geological Survey, p. 51–62. Summary groups, including their con- The Manitoba Geological Survey (MGS) began tained mineral occurrences; 1:20 000 scale mapping of the Bigstone Lake area in the and summer of 2016 as part of a renewed study of supracrustal • provide a revised assessment of mineral potential in rocks in the Bigstone–Wass–Knight lakes area. Results this part of the Superior province. of this field season, building partly on previous work, The Bigstone Lake area has been subject to sporadic included: 1) delineation of several ultramafic cumulates precious- and base-metal exploration activity since 1920, and related flows in the lower part of the greenstone belt including airborne and ground-loop electromagnetic sur- stratigraphy, with potential for Ni-sulphide mineraliza- veys, and has been tested by a total of 91 drillholes col- tion; 2) recognition of shallowing water depth during lared mostly along the southeastern shoreline. The largest emplacement of the volcanic stratigraphy, with potential exploration campaign was conducted by Noranda Explo- for precious-metal–rich volcanogenic massive sulphide ration Co. Ltd., from 1982 to 1986; the last submitted (VMS) deposits in the uppermost portions of the volcanic assessment report for this work recommended continued stratigraphy; 3) possible evidence of a major unconfor- drill testing of Zn and Au mineralization (Assessment File mity between rocks of the Hayes River and Island Lake 94359). Since 1986, no work has taken place at Bigstone groups; 4) definition of several regional alteration zones, Lake, apart from brief visits to a few outcrops as part of including mineral assemblages known elsewhere to occur a regional geochemical sampling program undertaken in in association with base-metal and/or shear-hosted gold deposits; and 5) recognition of a complex structural his- tory, including regional isoclinal and open folding fol- lowed by episodes of late shearing, with implications for understanding potential controls on vein-hosted precious- metal mineralization in the region. Introduction The Bigstone Lake area in east-central Manitoba (Figure GS-5-1) contains a wide range of Archean supra- crustal rock types, including ultramafic, mafic and felsic volcanic packages (Neale et al., 1986; Herd et al., 1987), and is considered to have potential for several types of mineral deposit. It includes a documented gold occur- rence, with reported values up to 72.3 g/t Au (Assessment File 94359, Manitoba Growth, Enterprise and Trade, Winnipeg), along with features potentially related to the formation of VMS deposits (e.g., zones of sphalerite and chalcopyrite mineralization, sodium depletion and chlo- rite alteration; Assessment File 94359). This document summarizes the results of field work Figure GS-5-1: Simplified geological domains of the undertaken in 2016 at Bigstone Lake. The work is part of northwestern Superior province, showing the locations of a project intended to selected greenstone belts (labelled) and the 2016 map- • document the supracrustal rocks of the Bigstone, ping area (outlined). Domain names are in italics and are Wass and Knight lakes area through systematic labelled as shown in Pilkington and Thomas (2001). Min- eral occurrences shown are mostly Au, Cu, Ni, Ag or Zn; shoreline mapping, supplemented by geochemical occurrences shown in the Island Lake greenstone belt and geochronological analyses; include two known gold deposits. Abbreviations: BLGB, • investigate regional stratigraphic and structural rela- Bigstone Lake greenstone belt; KWGB, Knight and Wass tionships between the Hayes River and Island Lake lakes greenstone belt; ILGB, Island Lake greenstone belt. Report of Activities 2016 51 1999 by Hudson Bay Exploration and Development Co. Local geology Ltd. (Assessment File 73614). Units documented during the 2016 field season are Geological mapping at Bigstone Lake was carried described here from oldest to youngest interpreted age. All of the rocks have been metamorphosed to upper- out in 1939 by MGS geologist R. McIntosh at 1:63 360 greenschist to lower-amphibolite facies; in the interest of scale (McIntosh, 1941), in 1974–1975 by geologists brevity, the ‘meta’ prefix is not used in this report. of the Geological Survey of Canada at 1:63 360 scale (Ermanovics, 1975; Ermanovics et al., 1975; Herd and Ermanovics, 1976) and in 1984–1985 by MGS geologist Lower stratigraphic sequence (Hayes River group) K. Neale at 1:20 000 scale (Neale, 1984, 1985; Neale et The oldest and lowermost supracrustal units of the al., 1986). Mapping by Neale (1984–1985) was mostly BLGB define the outer limbs of a belt-scale syncline limited to the eastern third of the 2016 mapping area. (Figure GS-5-2). The sequence consists mostly of mafic Detailed mapping at 1:10 000 scale was also undertaken to ultramafic intrusions and flows, with subordinate felsic in 1985 by Noranda geologists over a small portion of volcanic units and interflow sedimentary packages. the southeastern shoreline. The results of these mapping programs were used to inform the geological boundaries Lower mafic volcanic flows outside of the mapping area shown in Figure GS-5-2a, Basalt of the lower sequence is the most abundant particularly in areas inland. rock type in the BLGB, occurring along the outer belt Seven weeks of shoreline mapping at 1:20 000 scale margins in flow packages up to 5 km in apparent thickness was carried out during the summer at Bigstone Lake. A (Figure GS-5-2a). Variolitic pillowed flows make up two total of 670 stations were described, with 119 samples thirds of the unit and are typically grey-green on fresh and collected for geochemical, petrographic or geochronolog- weathered surfaces, aphyric and highly strained. On aver- ical analyses. Despite high water levels, excellent expo- age, pillows 20–80 cm thick are elongated (~4:1 aspect sures were encountered at several locations, mostly along ratios) along the regional S3 fabric, with significant local the central island chains. Inland access was particularly variations (up to 15:1) reflecting strain partitioning. Pil- difficult in the recently burned area along the southeastern low selvages 0.5–3 cm thick are generally well-preserved shore of Bigstone Lake (near the sphalerite occurrence and varioles 0.2–2 cm across, which coalesce toward pil- labeled in Figure GS-5-2b) and in densely forested areas low centres, make up between 50 and 80% of most out- around the southwestern tip of Bigstone Lake. crops. Massive, sparsely plagioclase-phyric basalt makes up the remaining third of the lower sequence mafic flows. Where least strained, the massive basalt contains ~20% Regional geology plagioclase phenocrysts <0.5 mm long and appears to transition into the gabbro unit described below. The Bigstone Lake greenstone belt (BLGB) is located in the northwestern Superior province, approxi- Concentric pillow fractures, finely bedded interpil- mately 70 km west of the Island Lake greenstone belt, low hyaloclastite, coarse interflow hyaloclastite-breccia layers, quartz-filled pillow shelves and rare quartz-filled and is contiguous with supracrustal rocks at Knight and amygdules up to 2 cm across were also documented in Wass lakes (Figure GS-5-1). Archean supracrustal rocks the flows. Shelves, amygdules and well-developed cusps of the Bigstone, Knight, Wass and Island lakes areas consistently indicate younging toward the belt centre (as share a generalized stratigraphy consisting of rocks of summarized in Figure GS-5-2b). the Hayes River group overlain by rocks of the Island Lake group. The Hayes River group is an arc-derived Subordinate components of the lower mafic flow sequence of mafic and minor ultramafic flows and unit, which are too small to display as separate units, intrusions transitioning to felsic flows and pyroclastic include interflow oxide-facies iron formation in layers rocks, which were (in the Island Lake region) emplaced <50 cm thick, mafic crystal-rich tuff (locally containing between ca. 2900 and 2861 Ma (Stevenson and Turek, zoned plagioclase up to 1 cm across) in rare beds up to a 1992). The Island Lake group is a sedimentary (±vol- few metres thick and rhyolite dikes <3 m thick. The rhyo- lite dikes are most abundant along the southeastern limb canic) sequence of basal conglomerate and marine of the BLGB, and are interpreted to be cogenetic with the turbidites, which were deposited ca. 2749–2729 Ma felsic volcanic and volcaniclastic units described below. (Neale, 1985; Herd et al., 1987; Stevenson and Turek, 1992). Late (post-Hayes River group) volcanic units are included in the Island Lake group described by Herd Felsic volcanic and volcaniclastic units et al. (1987), whereas Neale (1985) assigned them to a Felsic igneous rocks are intercalated with the lower (possibly unrelated) late supracrustal group. The supra- mafic volcanic flows in the eastern half of the BLGB, crustal units were intruded by granitoid plutons ranging both north and south of Bigstone Lake (Figure GS-5-2a). in age from ca. 2729 to 2699 Ma (Turek et al., 1986). Rhyolite to rhyodacite flows or domes make up about 52 Manitoba Geological