Fig. 1a,b. :
Massive sulphide ore from the Nova orebody, sawn and lightly polished on one side.
The most distinctive feature of this high-grade ore is the "loop texture" in the
sulphides, described by Barnes et al. (2020a,b). This sample is largely massive sulphides,
clove-coloured pyrrhotite enclosing coarse masses of paler, bronzey pentlandite, showing a few crystal faces, plus tarnished
yellow chalcopyrite. The sulphide loops are evident in the sawn face: pale pentlandite and yellow chalcopyrite in mm- to cm-scale rims around pyrrhotite domains.
Hand specimen data: weight 587.1 grams, 8.5x7.0x3.5 cm. Magnetic susceptibility is circa 107x10-3 SI units (range 99-115, n=6). At a first approximation, mode evident in hand specimen is very roughly 94 volume percent sulphides (70% pyrrhotite, 12% pentlandite, 12% chalcopyrite), 6% silicate crystals (green-black clinopyroxene?).
These samples come courtesy of Ben Cave, a geologist working for the Independence Group, at the Nova mine. The suite represent the remarkable sulphide ores of the Nova orebody, and are from Level 1915, Ore Drive K8, towards the WSW end of the small, sinuous host chonolith. This sample represents the massive sulphides and related breccias that infiltrate the granulite host rocks to the chonolith. No thin sections have been prepared at this time, and any errors in the sample descriptions are mine - GCW.
"Rock of the Month # 240, posted for June 2021" ---
20 Years of "Rock of the Month"!
Figs. 2-3. :
Coarse (cm-scale) blebs or clots of sulphides in the intrusion (left) and
finer disseminated sulphides (right). From the Nova orebody.
Hand specimen data. Left: clotted sulphides in host rock, weight 520.3 grams, 10x10x2 cm, Magnetic susceptibility is circa 21x10-3 SI units (range 13.5-27.5, n=6). Ore with circa 20 volume percent sulphides, mostly as coarse clots 5-10 mm in size (say, 12% chalcopyrite, 8% pyrrhotite and pentlandite). The host rock appears massive, silicate grain size 1-2 mm, apparently some 50% greenish-white plagioclase feldspar and 30% dark pyroxenes. To my untutored eye, a gabbronorite (?).
Right: disseminated sulphide ore, weight 612.5 grams, 14x9x2 cm. Magnetic susceptibility is circa 10x10-3 SI units (range 9.5-11.1, n=6). Here we have circa 12 volume percent sulphides (say, 8% pentlandite, 4% chalcopyrite) as scattered mm-scale grains, no large clots nor veins. The host lithology is some variant of ultramafic rock, perhaps a feldspar-bearing olivine websterite, but I don't "have my eye in" for these local exotic rock types! Silicates and sulphides are quite fine-grained, 2 mm or so.
The mafic-ultramafic intrusive rocks at the mine have been mapped as a mixture of cumulate -textured lithologies including olivine norite (OPX-PLAG plus OLIV), olivine gabbronorite (CPX-OPX-PLAG plus OLIV), olivine websterite (OPX-CPX plus OLIV) and lherzolite (OLIV plus OPX-CPX).
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:
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.
Barnes,SJ, Mungall,JE, Le Vaillant,M, Godel,B, Lesher,CM, Holwell,D, Lightfoot,PC, Krivolutskaya,N and Wei,B (2017) Sulfide-silicate textures in magmatic Ni-Cu-PGE sulfide ore deposits: disseminated and net-textured ores. Amer.Mineral. 102, 473-506.
Barnes,SJ, Taranovic,V, Miller,JM, Boyce,G and Beresford,S (2020a) Sulfide emplacement and migration in the Nova-Bollinger Ni-Cu-Co deposit, Albany-Fraser orogen, Western Australia. Econ.Geol. 115, 1749-1776.
Barnes,SJ, Taranovic,V, Schoneveld,LE, Mansur,ET, Le Vaillant,M, Dare,S, Staude,S, Evans,NJ and Blanks,D (2020b) The occurrence and origin of pentlandite- chalcopyrite- pyrrhotite loop textures in magmatic Ni-Cu sulfide ores. Econ.Geol. 115, 1777-1798.
Martin,L, Fiorentini,M, Barnes,SJ, Verrall,SJ and Beresford,S (2020) Potential origin(s) for garnet growth at silicate-sulphide contacts at Nova-Bollinger. 14th International Ni-Cu-PGE Symposium and Naldrett Memorial, Phase 2, virtual meeting (Holwell,D, Barnes,SJ and Schutesky,ME editors), abstract, 1p., 10-24 November.
Peters,B (2017) Geophysics for magmatic Ni-Cu-(PGE) exploration. Presentation to the Australian Society of Exploration Geophysicists, Perth, WA, on 18 October 2017, and available at the SGC web site (viewed 04 June 2021).
Standing,JG (2019) Structural setting and controls on Ni-Cu sulphide mineralisation at Nova-Bollinger, Fraser zone, WA. Australian Exploration Geoscience Conference, Perth, 3pp.
Graham Wilson, Draft posted 01-02 May 2021, minor amendments and additions to 15 June 2021
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