Published description and interpretation

The Nova-Bollinger Ni- Cu- Co deposit is located in Western Australia, some 700 km east of Perth, 220 km southwest of Kalgoorlie and 120 km east of Norseman. The ore deposit, composed of the Nova and Bollinger orebodies, was discovered by Sirius Resources in 2012. It lies in the Fraser Range orogen, near the southeast margin of the great Archean Yilgarn craton. The find now supports an underground mine with decline access, at a mining rate of 1.5 MT/year. The orebodies are hosted by an east-west, complex magma conduit or chonolith, intruding highly metamorphosed, granulite facies host rocks. Modern geophysical methods, with significant insights into the 2012 Nova-Bollinger discovery and its regional setting, were reviewed by geophysicist Bill Peters (2017).

The Nova-Bollinger deposits appear to be a new variant on normal magmatic sulphide mineralization. They represent the structural modification of magmatic sulphide ore: sulphide melt was mechanically extracted from the chonoliths and concentrated into structurally controlled positions in the footwall rocks. The ore-bearing intrusions were emplaced late in granulite facies regional metamorphism and deformation. The original magmatic melt was remobilized on foliations and into fold hinges, post-foliation pegmatites and a shear zone in footwall granulites. More "conventional" disseminated and net-textured sulphides remained, within the mafic-ultramafic chonolith rocks. The sulphide melt is thought to have remained mobile for up to 40 million years, during the Fraser-Albany orogeny (Standing, 2019).

The small chonolith was emplaced into lower crustal, granulite facies migmatite gneisses. Both disseminated and net-textured ores are found in the intrusion, and in addition there is a high proportion of massive, semi-massive and breccia exocontact ores, i.e., in the country rocks beyond the intrusion. Nova and Bollinger are two adjacent but spatially distinct orebodies with a combined metric resource estimated at 13.1 MT grading about 2% Ni, 0.8% Cu and 0.1% Co. Inside the chonolith, the endocontact ores show magmatic fabrics, such as net texture, leopard net texture (with cm-scale clots of olivine and intercumulus phases) and globular ores (some of which show association of sulphide blebs with clinopyroxene-carbonate intergrowths, perhaps infillings of CO₂-rich vapour bubbles). The sulphide melt invaded hot, soft country rock, the partial melting of which produced felsic leucosome, some rich in garnet (Barnes et al., 2020a). Garnet is ubiquitous in the metasediments and other granulite facies host rocks of the orebodies, which include massive sulphides. Garnet crystals at the interface of massive sulphides and silicate host rocks display variable crystal habits including skeletal crystals, corona textures and euhedral crystals (Martin et al., 2020).

Coarse chalcopyrite- pentlandite- pyrrhotite "loop textures" are typical of all ore types: this is taken to indicate formation of magmatic high-temperature pentlandite. The textures formed under extreme conditions of peak granulite facies regional metamorphism. The sulphides could have solidified in millions of years, cf. thousands of years for host silicate magmas (Barnes et al., 2020a). More generally, for a detailed, worldwide and beautifully illustrated review of magmatic sulphide ore textures and their perceived modes of genesis, see Barnes et al. (2017).

The best of the Nova mineralization is spectacular in both grade and in the ore textures evident in the coarse sulphides. Pentlandite is the dominant host mineral of Ni in most magmatic sulphide deposits and has been interpreted to form entirely by exsolution from mss, monosulphide solid solution, a sulphide mixture that concentrates in a magma chamber or conduit, as a liquid immiscible with the more voluminous host silicate magma. Upon cooling, individual sulphides such as the common pentlandite, chalcopyrite and pyrrhotite separate from the mss. During cooling, loops of pentlandite form, surrounding pyrrhotite. However, it has recently become clear that not all pentlandite forms by exsolution: some may form by reaction between mss and residual Ni and Cu -rich sulphide liquid, such that some pentlandite loops may be genuinely magmatic. The Nova-Bollinger orebody is a prime example. However, there are other examples in the literature, as elegantly displayed by a recent review (Barnes et al., 2020b). Other localities include the following:

• W.Australian deposits, e.g., Flying Fox, Perseverance, Spotted Quoll, and the Moran Shoot at Kambalda.
• Aguablanca in Spain.
• Noril'sk, Russia.
• Eagle, in the Midcontinent Rift of Michigan, USA.
• Voisey's Bay in Labrador, Newfoundland, Canada.

Now that this phenomenon has been so elegantly demonstrated in Australia, it may be expected that loop-textured ores will be recognized more elsewhere, not necessarily just in host intrusions that were emplaced into very hot lower-crustal settings. It may be recognized widely, as in the localities cited above. A further example is the Thunder Bay North intrusive complex, a suite of mafic-ultramafic intrusions emplaced into the late-Archean Quetico supracrustal belt of the Superior craton of the Canadian shield, in the Mesoproterozoic Midcontinent Rift of North America. A suite of small intrusions, some of chonolithic form, were emplaced at circa 1107 Ma, including Eagle (Michigan), Tamarack (Minnesota) and Thunder Bay North.

As a parting shot, let's consider mineral deposits in regions of intense lower-crustal, generally granulite facies regional metamorphism. Such deep crustal domains are often subject to partial melting and widespread development of migmatites and pegmatites. Deposits of gemstones, graphite and other minerals may result, as in Sri Lanka. Much of the metal endowment may be remobilized and lost, as is thought to be the case with gold deposits, leaving a few, generally smaller deposits preserved under specific conditions, such as Griffin's Find in W.Australia or Renco in Zimbabwe. Base metals may also be preserved, such as the striking, norite-hosted Cu deposits at O'okiep in Namaqualand, northwestern South Africa. In each case, the timing of mineralization (and/or magmatism) with respect to the peak of regional metamorphism will affect the nature of the resultant deposits. Some of the most refractory metals, of course, such as Cr in its principal economic source as chromite, may be found concentrated even at mantle depths, as at the Luobusha (Luobusa) ophiolite in southern Tibet.