The McFaulds Lake greenstone belt, in addition to its world-class chromite endowment, also hosts significant magmatic Ni-Cu-(PGE) mineralization at the Eagle's Nest deposit. Mineralization at Eagle's Nest occurs in a dynamic system that contains multiple generations of sulphide textures, consistent with its interpretation as a magmatic feeder among a network of ultramafic intrusions. Massive sulphides and abundant leopard net-textured, oikocrystic net-textured, and inclusion net-textured sulphide mineralization formed first and were consolidated enough to be cross-cut by late-stage pyroxenite, creating disrupted net-textured sulphide and minor late-segregating massive/semi-massive sulphide melts. Resolving the fundamental characteristics and genesis of mineralization at Eagle's Nest will provide critical information about the evolution of large chromium and Ni-Cu-PGE ore systems associated with Archean komatiitic magmatism within the Superior Province.

The Eagle's Nest Ni-Cu-(PGE) deposit occurs within the 2.73 Ga Esker intrusive complex of the Ring of Fire intrusive suite, in the McFaulds Lake greenstone belt of northern Ontario. Sulphide mineralization occurs along the northwestern margin of a subvertical ~500 m wide (north-south) x </-85 m thick (east-west) x >1500 m deep komatiitic ultramafic body composed of harzburgite+/-lherzolite+/-wehrlite that is interpreted to represent a structurally rotated, originally subhorizontal blade-shaped dyke. Disseminated (<15 wt% sulphide), net (15-35 wt% sulphide), and lesser semi-massive (50-80 wt% sulphide) and massive (>80 wt% sulphide) -textured sulphides have been characterized using microbeam scanning energy-dispersive X-ray fluorescence spectrometry (uXRF). The images produced by this process have identified the abundances and locations of Fe, Ni, Cu, Ca, Si, and Cr within different minerals in 11 samples with representative sulphide textures: disseminated, leopard net-textured (containing bimodal coarse- and fine-grained olivine), pinto net-textured (containing bimodal coarse orthopyroxene and fine olivine), inclusion net-textured (containing coarse inclusions of peridotite), disrupted net-textured (containing pyroxenite invading leopard and pinto net-textured sulphides), semi-massive (containing inclusions of peridotite, pyroxenite, gabbro, and net-textured sulphides), and massive. All sulphide textures typically contain magmatic assemblages of pyrrhotite-pentlandite-chalcopyrite-magnetite+/-chromite. Notable differences include small ellipsoidal chromite inclusions only in facies with disseminated textures, little to no clinopyroxene in leopard net-textures, abundant clinopyroxene in invasive pyroxenite in disrupted net-textures, high-Cr orthopyroxene oikocrysts in pinto net-textures, and variable degrees of segregation of Cu-rich residual sulphide liquid from fractionally crystallizing Fe-Ni-rich monosulphide solid solution, more in disrupted and inclusion net-textures than in disseminated textures or leopard or pinto net-textures. The wide variety of net textures, the abundance of inclusions, and the invasive pyroxenite indicate a complex history for the magmatic plumbing system. Inclusions and late-stage pyroxenitic melt may have physically facilitated migration of residual sulphide liquid from monosulphide solid solution, and the emplacement of the late pyroxenite may have slowed cooling and/or reheated the mineralization. Many of these details are less evident visually in slabbed cores or polished thin sections, highlighting the usefulness of uXRF in interpreting the genesis and beneficiation of texturally complex magmatic Ni-Cu-PGE ores.

The Eagle’s Nest Ni-Cu-(platinum group element; PGE) deposit occurs within the 2.73 Ga Esker intrusive complex of the Ring of Fire intrusive suite in the McFaulds Lake greenstone belt of northern Ontario. Mineralization occurs along the northern margin of a formerly ~500-m-high, ~85-m-thick, >1,500-m-long subvertical structurally rotated blade-shaped dike composed of harzburgite, lherzolite, and wehrlite. Three sulfide textural facies are present (percentage as proportion of total mineralization): (1) disseminated (~5%), (2) net texture (~80%), and (3) semimassive to massive (~15%). Five subfacies of net texture have been identified: (1) bimodal olivine-bearing leopard net texture (~50%), (2) inclusion net texture (~5%), (3) orthopyroxene-bearing pinto net texture (<1%), (4) localized zones of disrupted net texture (~30%) containing 3- to 5-cm-thick zones of barren amoeboid crosscutting pyroxenite, and (5) fine-grained patchy net texture (~15%). All textural facies are characterized by typical magmatic pyrrhotite-pentlandite-chalcopyrite-(platinum group mineral) assemblages. Massive sulfides are localized in two embayments along the basal contact separated by a topographic high, grading upward to rare semimassive, laterally more continuous net-textured, and disseminated sulfides, with gradational contacts between all textures except massive. Similar mean ore tenors of different sulfide textural facies (Ni100 ~7.5, 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 favorable for Ni-Cu-(PGE) mineralization.

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