GS-19 Geological investigations in the Bird River Sill, southeastern Manitoba (part of NTS 52L5): geology and preliminary geochemical results by C.A. Mealin1 Mealin, C.A. 2006: Geological investigations in the Bird River Sill, southeastern Manitoba (part of NTS 52L5): geology and preliminary geochemical results; in Report of Activities 2006, Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, p. 214–225. Summary and gravitational differentiation. In 2005, this study was initiated to examine the Bird Theyer (1985) identified unusual River Sill (BRS), a mafic-ultramafic layered intrusion cooling conditions requiring several magma pulses and located in the Bird River greenstone belt in southeastern cast doubt on a single magmatic intrusion theory. Manitoba. The BRS is currently an exploration target In 2005, this study was initiated to examine the for Ni-Cu–platinum group element (PGE) deposits and, controls on Ni-Cu-PGE mineralization and test the single in the past, hosted two Ni-Cu mines (Maskwa West and versus multiple injection hypothesis to establish the Dumbarton mines). relationship between mineralization and the magmatic Nickel-copper mineralization on the western half history. Specific objectives include of the BRS consists primarily of disseminated to blebby • detailed mapping of the Chrome, Page, Peterson pyrrhotite+chalcopyrite and is present in the ultramafic Block and National-Ledin properties (Figure GS-19-1); series of the Chrome and Page properties and the mafic • determination of the sulphur source for the Ni-Cu- series of the Wards property. During this study, several PGE mineralization; new sulphide-bearing localities in both the ultramafic and • delineation of the sill’s sulphur saturation history; mafic units of the sill have been identified. In addition, the geological contacts of the sill have been further • examination of the sill’s magmatic history; and constrained and the structural setting of the sill is being • examination of the influence of crustal contamina- redefined (this study and Duguet et al., GS-16, this vol- tion/assimilation on the sill. ume). This report presents the geology of the western half This report includes new geological data for the BRS of the sill and preliminary geochemical results. and preliminary geochemical results. Previous work Introduction Between 1982 and 1984, P. Theyer and assistants The Bird River sill (BRS) is a mafic-ultramafic sawed three semicontinuous sampling sections (western, intrusion located in the southern part of the Bird River central and eastern cuts) across the Chrome property Domain, approximately 120 km northeast of Winnipeg in in the BRS. The western and eastern cuts crosscut the southeastern Manitoba. Since the 1920s, the sill has been ultramafic portion and the central cut the gabbroic known to contain Ni, Cu and Cr mineralization, and has portion of the sill. These provide a nearly continuous hosted two Ni-Cu mines, the Dumbarton mine (from 1969 channel sample and cross-section of the sill. Detailed to 1974) and the Maskwa West mine (from 1974 to 1976). geochemical analyses led to the identification of two The presence of platinum group elements (PGE) was PGE- and sulphide-enriched layers within the ultramafic discovered in the early 1980s by Theyer (1985). unit (Theyer, 1985). Additional PGE-bearing layers were Numerous studies have produced different theories identified in the following years (Scoates et al., 1989). over the years on the emplacement of the BRS and its Theyer (1985) defined several distinct igneous cycles inherent Ni-Cu-PGE and Cr mineralization. The sill was based on remnant crystal morphology of olivine within considered by Bateman (1943) to be the result of two the eastern and western cuts. The base of an idealized different magma pulses: the first being of ultramafic magmatic cycle is defined as a layer of fine-grained, composition and the second of mafic composition. tightly packed, lensoid-stage olivine that exhibits an Osborne (1949), the first to describe the magmatic lithology abrupt lower contact with skeletal olivine. The remnant of the sill in detail, proposed a complex multi-injection olivine textures exhibit a repeated progression from theory. Juhas (1973), Trueman (1971) and Scoates (1983) simple, ovoid crystal habits to increasingly complex all suggested a single magmatic injection where the dendritic crystals (Theyer, 1985). layering was formed by subsequent crystal fractionation 1 University of Waterloo, Waterloo, Ontario N2L 3G1 214 Manitoba Geological Survey Figure GS-19-1: Location of the Bird River Sill (after Theyer et al., 2001). Talkington et al. (1983) completed a study on plati- English River Domain and to the south by the Winnipeg num group mineral (PGM) inclusions in chromite from River Domain. the Chrome property of the BRS. Platinum group mineral The supracrustal rocks of the BRGB have been inclusions in chromite occur as discrete grains of laurite subdivided into six formations (Trueman, 1971; Cerny et (Ru, Os, Ir)S2, rutheniridosmine (Os, Ir, Ru alloy) and one al., 1981): PGE alloy containing 0.96 wt. % Rh (Talkington et al., Eaglenest Lake Formation: volcaniclastic sedimentary 1983). Laurite grains contained up to 2.99 wt. % Pd, but rocks and mafic lavas no Pt was detected. All the chromite specimens collected contained inclusions of silicate minerals and hematite, but Lamprey Falls Formation: pillowed basalt with interca- only samples from chromite seams contained PGM inclu- lated tuff and iron formation sions. Increasing oxygen fugacity by wallrock assimila- Peterson Creek Formation: rhyolite flows, pyroclastic tion or new magma injection is thought to be the trigger breccia, lapilli tuff, tuff and volcanic sandstone for the chromite precipitation (Talkington et al., 1983). Bernic Lake Formation: basalt, andesite, dacite, rhyo- lite, iron formation, conglomerate and sandstone Regional geology Flanders Lake Formation: polymictic conglomerate The BRS intrudes the east-trending Bird River green- and metamorphosed lithic arenite stone belt (BRGB), located in the southern portion of the Booster Lake Formation: greywacke-mudstone turbi- Bird River Domain. The BRGB is in the Archean Supe- dite, interbedded conglomerate and iron formation rior Province of the Canadian Shield and is located in Further descriptions can be found in Duguet et al. (GS-16, southeastern Manitoba. It is bounded to the north by the this volume). Report of Activities 2006 215 Geology of the Bird River Sill the Chrome, Page and Peterson Block properties. The The BRS is a differentiated, layered, mafic-ultramafic shear zones are mineralized with remobilized chalcopy- sill that intrudes the Lamprey Falls Formation and is rite, pyrite and pyrrhotite. Further petrographic study is overlain by the Peterson Creek Formation. It trends ongoing and geochemical analyses are pending. easterly and is reported to be 2744.7 ±5.2 Ma (Wang, Davies (1952) mapped numerous east-trending 1993). The sill has been metamorphosed to greenschist gabbroic intrusions and one granitic intrusion in the facies, and locally by contact metamorphism to lower Lamprey Falls Formation north of the BRS. The gabbro is amphibolite facies adjacent to granitic intrusions (Juhas, massive and relatively undeformed. These intrusions were 1973). Primary textures are generally well preserved, mapped by Juhas (1973) as being part of the BRS, but although metamorphic minerals replace most of the they have since been treated as separate from the sill and original minerals (Theyer, 2002). are completely omitted from later compilations (Cerny et Mapping the western half of the BRS and immedi- al., 1981). The nature of the gabbroic intrusions, their age ate surrounding formations of the BRGB at a scale of and their relation to the sill are currently unknown, but 1:5000 was completed over the 2005 and 2006 summer they have been sampled for further investigation. field seasons. The final map (Mealin, 2006) is a compila- tion of this author’s preliminary BRS map, Davies’ (1952) Peterson Creek Formation BRGB map, Williamson’s (1990) Chrome property The BRS is overlain by the Peterson Creek Forma- maps, Duguet et al.’s (GS-16, this volume) BRGB map and tion. The formation overlying the BRS has historically interpretations from total field magnetic data (made avail- been interpreted to be part of the Bernic Lake Formation, able by Gossan Resources Ltd. and North American but recent work by Duguet et al. (GS-16, this volume) Palladium Ltd.). has redefined the formations in this part of the belt. The Peterson Creek Formation comprises metavol- Relationships between the Bird River Sill and canic and metasedimentary sequences. The volcanic country rock types rocks include rhyolite, lapilli tuff and tuff. The sedimen- Lamprey Falls Formation tary rocks comprise greywacke turbidite, chert, mafic The BRS is hosted by the Lamprey Falls Formation. clast conglomerate and a polymictic conglomerate. The The intrusive contact is exposed in outcrops on the polymictic conglomerate was identified overlying the Chrome, Peterson Block and National-Ledin properties sill south of the Chrome, Peterson block and National- (Figure GS-19-2b). The formation comprises massive to Ledin properties. This conglomerate contains gabbroic pillowed basalt that ranges from aphyric to megacrystic. clasts that are believed to be from the mafic unit of the The pillowed basalt is generally moderately deformed and BRS. Therefore, the contact between the Peterson Creek locally brecciated. Gilbert (2005) demonstrated that the Formation