Summary:
Cosmos’ nickel sulphide deposits form part of the Cosmos Nickel Complex, which lies within the Agnew-Wiluna Belt of the central Yilgarn Craton.
The genetic formation model for the nickel mineralisation is the ‘Kambalda-style’ model where precipitation of nickel sulphides is interpreted to occur in channelised komatiitic lava flows and/or lava tubes. This model proposes two dominant deposit types – Type 1 and Type 2.
Type 1 deposits are interpreted to have formed where the hottest lava in the centre of a lava channel thermally erodes into a sulphidic substrate and massive sulphides accumulate at the channel’s base. This high-grade primary formation style is applicable to the Cosmos, Cosmos Deeps, AM1, AM2, parts of AM5 and the Odysseus Massive zones.
Type 1 deposits are often subject to mobilisation into structural sites when massive sulphide bodies behave plastically or even re-melt under the high metamorphic temperatures and pressures that are generated by local or regional tectonic faulting and/or folding events. The Cosmos Deeps and the Odysseus Massive zone are examples of this remobilisation style.
Type 2 deposits are interpreted to have formed from cooler and slower-flowing lavas than those for Type 1 deposits. In this model, the mineral olivine is envisaged to crystallise from these cooler lavas, with the olivine grains settling to the channel’s base shortly after the precipitation of sulphides. In this model, the coeval sulphides crystallise between the olivine grains, giving rise to a disseminated to ‘matrix’ textured nickel sulphide mineralisation that builds up from the channel base. This style of mineralisation is applicable to the AM5, AM6 and Odysseus disseminated zones at Cosmos.
The sulphide nickel assemblages at Cosmos are ‘high tenor’, meaning that the sulphides are dominated by the nickel-bearing mineral pentlandite. The sulphide assemblages also contain the sulphide mineral pyrrhotite, with minor amounts of pyrite and chalcopyrite, and in some places, small concentrations of rarer nickel sulphides such as valleriite and millerite.
The mineralisation typically occurs in association with the basal zone of high magnesia (MgO) cumulate ultramafic rocks.
Odysseus has two discrete zones of mainly disseminated nickel sulphide mineralisation, known as Odysseus North, which is centred at coordinates 27°35'48"S and 120°34'51"E, and Odysseus South, which is centred about 400m south southwest of Odysseus North’s midpoint.
The geology of Odysseus is that of a typical WA komatiite-hosted nickel sulphide deposit with mostly thick disseminated sulphides but with a zone of massive (likely remobilised) massive sulphides below and south of Odysseus North. The rocks have undergone amphibolite grade metamorphism and are folded and faulted – structural geological work is ongoing to assess this aspect of the deposit. The ultramafic mineralisation host has been disrupted by faults and nickel-barren felsic pegmatite dykes have intruded the area, likely along fault structures, with the pegmatites both expanding the volume and/or assimilating the pre-existing rocks.
Odysseus South low grade outer low grade zone:
The strike length of is about 450m. The vertical height from the top to its base is about 300m. The width of the between 0.8 m and 120m and averages 35m. The average grade and thickness increases down plunge to the north.
Odysseus North outer low grade zone:
The strike length is about 420m. The vertical height from the top of the low grade mineralisation to its base is about 100m. The width of mineralisation varies between 0.8m and 71m and averages 28m (after application of a minimum 5m threshold). The average grade and thickness increase to the north.
Both zones start at about 1,000m below surface.
The top of AM5 is about 700m below surface and centred on coordinates 27°36'21"S and 120°34'31"E and is 350m down dip from the lower limits of Mt Goode's low grade disseminated mineralisation. The base of AM5's magmatic nickel sulphide mineralisation coincides with the base of Cosmos’ lower ultramafic unit. The mineralisation comprises two sub-parallel, steeply dipping and plunging lenses that are separated by a felsic volcanic unit. AM5’s massive sulphide mineralisation is interpreted to have been originally of basal primary style but has undergone subsequent folding and thrusting. The massive sulphide mineralisation has an average thickness of only one metre, but in some tectonically induced overlaps, the average thickness increases to approximately 4m. The longest downdip distance is about 500m and the top of AM5, which starts at about 600m below surface. The mineralised width is variable and ranges between10m and about 120m.
The AM6 is centred on coordinates 27°36'31"S and 120°34'28"E, which is 300m south-southwest of AM5’s centre. AM6 has a strike extent of about 400m and dips about 75° towards the east with a down dip extent of about 250m. AM6’s disseminated mineralisation ranges from 2 to 25m in true thickness. The longest downdip distance is about 300m and the top of the deposit is about 900m below surface. Width is variable and ranges from about 10 to 40m.
The geometry and dip of AM6’s mineralisation are both influenced by multiple northeast-trending faults which truncate the AM6 mineralisation at its northern and southern extents. Like AM5 and Odysseus, younger nickel-barren pegmatite dykes cross cut the mineralisation, albeit within a lower spatial frequency and volume than occurs at Odysseus.
Production
On 6 November 2023, IGO Ltd. re-started processing operations at Cosmos - Odysseus (was on care and maintenance since 2012), with its partly new and partly refurbished nominally 1.1Mt/a sulphide concentrator and supporting infrastructure.
In FY24, Cosmos - Odysseus was mined on a very limited basis prior to transitioning in care and maintenance in May 2024.
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Nickel
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Nickel
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