Variations in the textural facies of sulphide minerals in the Eagle's Nest Ni-Cu-(PGE) deposit, McFaulds Lake greenstone belt, Superior Province, Ontario: insights from microbeam scanning energy-dispersive X-ray fluorescence spectrometry

Abstract

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.

Diversity of Net-Textured Sulfides in Magmatic Sulfide Deposits: Insights from the Eagle’s Nest Ni-Cu-(Platinum Group Element) Deposit, McFaulds Lake Greenstone Belt, Superior Province, Canada

“…Many of the sulfide facies at Eagle's Nest (Table 1) are similar to those present in many other komatiite-associated Ni-Cu-(PGE) deposits (e.g., Alexo : Naldrett, 1966;Kambalda: Ewers and Hudson, 1972;Raglan: Lesher, 2007;Bazilevskaya, 2009; see review by Barnes et al, 2017), but there are several types that have been not been described previously (e.g., pinto net) or are much less common (inclusion net). Each of the textures in the deposit described below, based on core logging, petrography, and sulfur contents, is shown schematically in Figure 3, and representative samples are illustrated in Figures 4-9 (Zuccarelli et al, 2020).…”