Sulfide Emplacement and Migration in the Nova-Bollinger Ni-Cu-Co Deposit, Albany-Fraser Orogen, Western Australia

Abstract

The Nova-Bollinger Ni-Cu sulfide deposit is associated with a small chonolith (tube-shaped) intrusion emplaced at lower crustal depths into granulite facies migmatite gneisses. The deposit comprises disseminated and net-textured ores within the intrusions and a high proportion of massive, semimassive, and breccia exocontact ores within the underlying country rocks. Internally disposed endocontact ores show typical magmatic textures including conventional net texture, leopard net texture characterized by the presence of centimeter-sized clots of olivine and intercumulus phases, and globular ores. Some of the globular ores show an association of sulfide blebs with clinopyroxene-carbonate intergrowths that may represent infilling of original CO2-rich vapor bubbles. The exocontact ores have an assemblage of textures indicative of emplacement into hot, soft country rocks at a large-scale melting-infiltration front. Characteristic features range from hard-walled extensional vein arrays to complex infiltrations of disseminated sulfide within chaotically folded paragneiss. Sulfide infiltration was accompanied by partial melting of the country rock, producing felsic leucosomes, some of them strongly enriched in garnet, mainly occupying vein walls and interpreted as the result of counterflow of displaced silicate partial melt. Coarse-grained pentlandite-chalcopyrite-pyrrhotite loop textures are characteristic of all ore types, down to the scale of the infiltrating sulfides within the gneisses, and are regarded as diagnostically magmatic textures generated by sulfide liquid fractionation and growth of high-temperature pentlandite by peritectic reaction between fractionated sulfide melt and early crystallized monosulfide solid solution. The highly distinctive features of the Nova-Bollinger ores are a consequence of their emplacement in the mid to lower crust under peak granulite facies conditions. Under these unusual conditions the timescales for cooling between the silicate solidus and sulfide solidus temperatures were of the order of millions of years, being controlled by the temperature-time path for the exhumation of the orogen as a whole. Sulfides solidified over a time period three orders of magnitude greater than the thousand-year timescale for the solidification of the host silicate magmas. Furthermore, timescales for deformation matched those for cooling and solidification, allowing the country rocks to undergo deformation during ore emplacement. Fluctuating strain rates during and after initial emplacement of the carrier magmas into the host intrusion caused episodes of brittle extension, allowing unusually efficient penetration of partially molten sulfide into heterogeneous, partially molten silicate country rock, resulting in an unusually extensive thermomechanical aureole compared with other mafic intrusion-hosted nickel systems globally.

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

“…Inclusion net-textured sulfide: This texture has been described at Nova by Barnes et al (2020) but appears to be rare to absent in other deposits. It contains sporadic and unevenly distributed coarse, ellipsoidal peridotite and pyroxenite anteliths, which were likely incorporated upstream in the plumbing system.…”

“…Leopard net texture: This contains fine-grained subhedral olivine phenocrysts and coarse-grained ellipsoidal serpentinized olivine-(chromite) aggregates. Similar aggregates are present at Sakatti (Brownscombe et al, 2015) and Nova (Barnes et al, 2020), and their origin is not well understood. The difference in the mineralogy of the phenocrysts and aggregates at Eagle's Nest suggests that the former crystallized from a higher-Mg magma that was saturated only in olivine, whereas the latter crystallized from a lower-Mg magma that was saturated in olivine-chromite (Murck and Campbell, 1986;Barnes, 1998).…”

Nickel in olivine as an exploration indicator for magmatic Ni-Cu sulfide deposits: A data review and re-evaluation

“…A distinctive plagioclase-bearing lherzolite containing both orthopyroxene and olivine as cumulus phases is a characteristic of the Lower Intrusion and the basal series of the Upper. The intrusions were emplaced under peak metamorphic conditions at around 20-25 km depth into granulite facies paragneisses which underwent partial melting during extensive infiltration of sulfides into the footwall (Barnes et al, 2020). The geometry of the intrusions and the disposition of the sulfides suggest that the Lower intrusion formed as an apophysis off the base of the Upper, propagating westward into the footwall rocks (Taranovic et al, 2021).…”

Preservation of basement architecture in zircon Hf isotope maps

“…However, these techniques have not been tested in younger terranes such as the Proterozoic orogenic belts that girdle many Archaean cratons; many of which were subject to high‐ T metamorphism (Harley, 1992) and potentially host a greater variety of magmatic source rocks and melt generation processes owing to the emergence of plate tectonics across the Archaean–Proterozoic transition (Keller & Schoene, 2012). The Albany–Fraser Orogen (AFO) is a Proterozoic orogenic belt that wraps around the southern and eastern margin of the Yilgarn Craton in Western Australia, and is relatively well‐endowed in economic metal deposits such as the Tropicana gold deposit and Nova–Bollinger Ni–Cu magmatic sulphide deposit (Barnes et al, 2020; Crawford & Doyle, 2016). In this paper, we use U–Pb, Lu–Hf and O isotopes on inherited and magmatic zircon crystals from granites of the AFO, to image the modification of this ancient craton margin over both space and time, and test whether Hf isotopes are able to image mineralisation pathways along reworked craton margins.…”

“…Archaean Yilgarn Craton variations in Hf isotope compositions of zircon have been inferred to be a proxy for lithospheric thickness based partly on the spatial correlation between juvenile Hf isotope signatures, komatiite abundance, and base-metal fertility, which are typically associated with regions of lithospheric extension(Mole et al, 2014). In time slices 3 and 5 (Figure2c,d), clusters of base-metal mineralisation in the AFO are associated with relatively juvenile zircon Hf isotope signature focussed around the southern part of the Fraser Zone, and are associated with maficultramafic rocks(Barnes et al, 2020). However, similar base-metal clusters are also associated with relatively evolved Hf isotope signatures in high-grade gneisses in the Tropicana zone (Figure3).…”