Sulfide textures, geochemistry, and genesis of the Komatiite-Associated Eagle’s Nest Ni-Cu- (PGE) Deposit, McFaulds Lake Greenstone Belt, Superior Province, Ontario, Canada

Abstract

The Eagle’s Nest nickel-copper-(platinum-group element) deposit occurs within the 2.73 billion- year-old Double Eagle intrusive complex of the Ring of Fire intrusive suite, in the McFaulds Lake

greenstone belt in northern Ontario. It contains 11 million tonnes of proven and probable mineralization grading 1.68 wt.% nickel, 0.87 wt.% copper, 0.89 g/t Pt, 3.1 g/t Pd, and 0.18 g/t Au, and 9 million tonnes of inferred mineralization grading 1.0 wt.% nickel, 1.14 wt.% copper,

1.16 g/t Pt, and 3.49 g/t Pd. Mineralization occurs along the northern margin of a subvertical blade- shaped dike~500 m wide (N-S) x ~85 m thick (E-W) x >1500 m deep composed of harzburgite,

lherzolite, and wehrlite. It is interpreted to have been emplaced as a subhorizontal blade-shaped intrusive body into tonalitic rocks below the Double Eagle intrusion and rotated into its present subvertical orientation. Four sulfide textural facies have been defined spatially and geochemically through detailed core logging, petrography, and whole-rock and mineral geochemical analyses: 1) disseminated (fine-grained olivine and pyroxene with <15 wt % uniformly distributed interstitial sulfide), 2) net (fine-grained olivine and minor pyroxene with 15-35 wt% semi-continuously to continuously distributed interstitial sulfide), 3) semi-massive (50-80 wt% sulfide with 20-40% anteliths and gabbroic xenoliths), and 4) massive (>80 wt% sulfide with <20% xenoliths). There are relatively few areas containing 35-50 wt% sulfide mineralization. Five subfacies of net texture

have been identified: 1) bimodal olivine bearing leopard-net texture (most common), 2) fine- grained patchy-net texture with irregular patches of sulfide (uncommon), 3) orthopyroxene- bearing pinto-net texture (coarse altered orthopyroxene similar to white spots on pinto-coloured

horse), 4) inclusion-net texture, each characterized by different peridotite inclusion populations,

and 5) localized zones of ‘disrupted-net texture” containing 3-5 cm thick zones of amoeboid cross- cutting barren pyroxenite. All textural facies are characterized by a typical magmatic sulfide

assemblage of pyrrhotite – pentlandite – chalcopyrite with minor platinum-group minerals. Massive sulfides are localized in two embayments along the basal contact that are separated by a topographical high, grading upwards to rare semi-massive, laterally more continuous net texture, and disseminated sulfides, with gradational contacts between textures except with massive sulfide. The similar mean ore tenors of the different sulfide textural facies, Ni100 ~7.5 and Cu100 ~4.8, suggest that the majority of the mineralization formed from similar magma compositions at similar magma:sulfide ratios, but the presence of different inclusion populations (peridotite, gabbro, chromitite) and the presence of disrupted-net texture indicates that the olivine, inclusions, and sulfide melts accumulated from multiple pulses in a dynamic system. The smaller, blade-shaped, sulfide-rich, chromite-poor Eagle’s Nest body does not appear to be the feeder to the overlying larger, oblate, sulfide-poor, chromite-rich Double Eagle body. This highlights the need to understand the fluid dynamics of entire plumbing systems when exploring for these deposit types and the significance of smaller, more dynamic magmatic conduits as environments favourable for nickel-copper-(platinum-group element) mineralization and of larger, less dynamic chonoliths/sills, as environments favourable to chromite mineralization.