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Mineral Chemistry of Chromite in the Mayville

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Mineral Chemistry of Chromite in the Mayville GS-3 Mineral chemistry of chromite in the Mayville intrusion: evidence for petrogenesis and linkage to the Bird River sill in the Neoarchean Bird River greenstone belt, southeastern Manitoba (parts of NTS 52L5, 6, 12) by X.M. Yang and H.P. Gilbert Yang, X.M. and Gilbert, H.P. 2014: Mineral chemistry of chromite in the Mayville intrusion: evidence for petrogenesis and linkage to the Bird River sill in the Neoarchean Bird River greenstone belt, southeastern Manitoba (NTS 52L5, 6, 12); in Report of Activities 2014, Manitoba Mineral Resources, Manitoba Geological Survey, p. 32–48. Summary Ni-Cu-PGE mineralization, in In 2014, the Manitoba Geological Survey com- accord with the widespread occurrence of contact-style pleted bedrock mapping in the northern arm of the Bird magmatic sulphide mineralization near the base of the River greenstone belt of the western Superior province, heterolithic breccia zone (HBZ) and at the basal contact southeastern Manitoba. The project aim was to provide of the intrusion. Furthermore, the MI is prospective for fundamental geoscientific information to assist mineral reef-style, stratiform PGE mineralization in transitional exploration, in particular for magmatic Ni–Cu–platinum and/or contact zones between major rock units, and for group element (PGE)–Cr mineralization associated with stratiform chromitite mineralization as demonstrated by mafic–ultramafic intrusions in the region. Because mafic– the presence of massive chromitite bands and disrupted ultramafic rocks are readily altered by deformation, meta- chromitite-pyroxenite layers within the HBZ. morphism, magmatism and related hydrothermal systems, their primary mineral assemblages and geochemical com- Introduction position are typically modified, requiring more robust In 2014, the Manitoba Geological Survey (MGS) geochemical techniques to ‘see through’ these effects. continued the multiyear mapping project in the north- Consequently, the mineral chemistry of chromite, which ern arm of the Bird River greenstone belt (BRGB) of the is highly refractory and resistant to chemical modifica- western Superior province in southeastern Manitoba (Fig- tion, has been examined to help evaluate the affinity and ure GS-3-1). This project has been conducted by the MGS fertility of mafic–ultramafic systems in the Bird River in collaboration with the Geological Survey of Canada belt. (GSC) through a Targeted Geoscience Initiative 4 pro- This report presents chemical composition data on gram, which was supported by Mustang Minerals Corp. chromite from the Neoarchean Mayville mafic–ultramafic (MMC). The main objectives of the project are to intrusion (MI) in the north arm of the Bird River green- • update the regional bedrock map in the Cat Creek and stone belt, and compares them to that of chromite in the Cat Lake–Euclid Lake areas; Bird River sill (BRS) in the southern limb of the belt, in • address the geological evolution and geodynamic order to provide insight into the magmatic affinity, mag- environment of the BRGB; and matic processes, tectonic setting and petrogenetic rela- tionships. Chromite in the MI is characterized by higher • assess the metallogeny of magmatic Ni-Cu-PGE sul- Fe-number (Fe# = 100 x Fe2+ / [Fe2+ + Mg2+]) and Al/Mg phide and Cr mineralization associated with mafic– ratio, variable Cr-number (Cr# = 100 x Cr3+ / [Cr3+ + Al3+]) ultramafic intrusions within the BRGB. and lower Mg-number (Mg# = 100 x Mg2+ / [Mg2+ + The northern arm of the BRGB, including the May- Fe2+]) compared to that in the BRS. This indicates that ville mafic–ultramafic intrusion (MI) and surrounding the parent magma(s) to the MI is/are of tholeiitic affinity, rocks in the Cat Creek area, were mapped by the MGS like those of the BRS but more evolved, and may have in 2012 at 1:12 500 scale. This bedrock geology map has been derived from subcontinental aluminous lithospheric been revised, expanded and upgraded to 1:10 000 scale mantle enriched by slab melt(s) related to plate subduc- (Yang, 2014) based on data acquired in the last two years tion. In contrast, the parental magma(s) to the BRS was/ (Figure GS-3-2). The MI is a composite layered intrusion, were less evolved and may have been sourced from more comprising dominantly gabbroic–anorthositic rocks with depleted subcontinental lithosphere mantle. Although the minor ultramafic components, which was intruded into MI is synchronous with the BRS and both are thus related mid-ocean-ridge basalt (MORB)–type basalts overlying to the extensive ‘Bird River magmatic event’ proposed by Mesoarchean basement represented by the Maskwa Lake Houlé et al. (2013), they are interpreted to result from dif- batholith I (Yang et al., 2012, 2013). Bedrock mapping ferent petrogenetic processes that reflect either different and exploration drilling indicate that Ni-Cu-PGE-Cr min- sources or degrees of differentiation. This study comple- eralization is hosted mostly in the mafic–ultramafic units ments detailed bedrock mapping and lithogeochemical in the lower part, and/or at the base, of the MI. Since the investigations, and indicates that the MI is fertile for MI underwent regional metamorphism and was intruded 32 Manitoba Geological Survey Trans Licence road Mayville intrusion Cat Lake 313000E Maskwa Lake 5606000N Euclid Lake 314 Euclid Maskwa Lake batholith Springer Geological contact Lake (approximate) Fault 315 Provincial road National Gravel road, track Ledin Page A Chrome A Mine (operational, Maskwa Bird Lake abandoned) Dumbarton North Winnipeg River Bird Lake NWRSZ A A shear zone 315 Bird River Booster SWRSZ South Winnipeg River sill Lake shear zone Flanders Lake Coppermine Creek Bernic Lake River Bird Birse Shatford Creek Shatford Lac Lake Lake du NWRSZ Bonnet Birse Lake Poplar pluton Bay (Wards) SWRSZ Lac du Bonnet batholith 315 343429E Pinawa 0 10 30 km Winnipeg River kilometres 5578014N Bird River subprovince English River subprovince INTRUSIVE ROCKS VOLCANIC AND SEDIMENTARY ROCKS Paragneiss, granitoid intrusive rocks, Pegmatitic granite BIRD RIVER BELT SOUTH PANEL BIRD RIVER BELT NORTH PANEL CAT LAKE AREA migmatite, pegmatite Bernic Lake formation Diverse arc assemblage Sedimentary and Granite, granodiorite, volcanic rocks, Winnipeg River subprovince tonalite Heterolithic volcanic Massive to fragmental, related gneiss breccia, rhyolite, mafic to felsic volcanic Tonalite, granodiorite, Gabbro, diorite, quartz basalt, andesite granitoid gneiss and sedimentary rocks Tholeiitic basalt diorite Eaglenest Lake formation Peterson Creek formation Pyroxenite, anorthosite, Ni-Cu-PGE deposit/occurrence gabbro Greywacke, siltstone Massive to fragmental felsic volcanic rocks Cr-PGE deposit/occurrence LATE SEDIMENTARY ROCKS Southern MORB-type formation Northern MORB-type formation Flanders Lake formation Basalt, aphyric; gabbro Basalt, aphyric; gabbro Arenite, polymictic conglomerate Booster Lake formation Greywacke, siltstone Figure GS-3-1: Regional geology between Cat Lake–Euclid Lake and the Winnipeg River, southeastern Manitoba, show- ing the southern arm of the Bird River greenstone belt between Lac du Bonnet and Flanders Lake, and the northern arm extending as far as the Mayville intrusion. Locations of chromite deposits (e.g., Chrome property, Euclid Lake) marked by black stars. Report of Activities 2014 33 5615000N 322500E Maskwa River No outcrop Cat Creek Cat Creek Maskwa Lake road River Maskwa 0 2 Donner kilometres Lake 309000E 5608000N Granitoid intrusive rocks Pegmatite Leucogabbro Geological contact Muscovite/garnet-bearing granodiorite, granite Gabbroic anorthosite to anorthosite Fault Tonalite, trondhjemite, granodiorite Heterolithic breccia Limit of mapping Maskwa Lake batholith granitoids Basal mafic–ultramafic rocks Gravel road Mayville mafic–ultramafic intrusion Metasedimentary, metavolcanic and related intrusive rocks Trail Quartz diorite to tonalite Lithic greywacke, siltstone, conglomerate Diabasic/gabbroic rock MORB-type basalt Gabbro: medium grained Gabbro: fine to medium grained Figure GS-3-2: Simplified geological map of the Neoarchean Mayville intrusion in the northern arm of the Bird River greenstone belt, southeastern Manitoba (modified after Yang, 2014). A quartz diorite to tonalite (unit 10) dike (or dikes) within the Mayville intrusion is/are indicated by the orange dot (not to scale) at the end of the black arrow. by Neoarchean tonalite, trondhjemite and granodiorite presented, to show the important role played by this min- (TTG; e.g., Peck et al., 2002; Yang and Gilbert, 2014), eral in investigations of petrogenesis of mafic–ultramafic its primary mineral assemblages and lithogeochemical rocks and in mineral exploration. characteristics were strongly modified, although mag- matic textures are generally well preserved (Peck et al., Brief review of chromite studies 1999; Yang et al., 2011, 2012). Due to its highly refrac- Chromite (Fe2+Cr O ) is an end member of the spinel tory nature, chromite in the MI provides a record of the 2 4 group of minerals, whose formula is AB O (where A is primary geochemical signatures of the magmas, which 2 4 Fe2+, Ni2+, Mn2+, Co2+ or Zn2+ in four-fold co-ordination are essential in evaluating its fertility for Ni-Cu-PGE-Cr T sites and B is Cr3+, Fe3+, Al3+ or Ti4+ in six-fold co- mineralization and for exploration targeting. ordination M sites; Deer et al., 1962, 1992). Spinel group This report concentrates on the Neoarchean MI minerals have long been used as ‘petrogenetic indicators’ (2742.8 ±0.8 Ma; see Houlé et al., 2013) by using chro- (Irvine, 1965, 1967), because 1)
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