Summary:
Cosmos’ cluster, or ‘camp’ of nickel sulphide deposits, occurs within or adjacent to a local sequence of ~2.7 billion year (Ga) old (Archean age), and now metamorphosed, komatiitic lavas that are part of the Agnew-Wiluna Greenstone Belt (AWGB) of the Kalgoorlie Terrane of the Eastern 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 (Pe). The sulphide assemblages also contain the sulphide mineral pyrrhotite (Po), with minor amounts of pyrite (Py) and chalcopyrite (Py), and in some pla
The mineralisation typically occurs in association with the basal zone of high magnesium oxide (MgO) cumulate ultramafic rocks.
ODS occurs within the Cosmos ultramafic and is located approximately 1,000-1,100m below surface. The AM5 and AM6 zones are both situated to the south and are thought to be genetically related to Odysseus given they all exhibit similar mineralisation characteristics (grade, tenor and sulphide type)
ODS is an elongated, oblate tubular body that strikes north-south (355°) for over 330m, dips 45° east and plunges -5° to the north. Sulphide mineralisation is best concentrated 20-30m above the footwall and consists of highly disseminated to net-textured pentlandite-pyrrhotite in the central and basal zones and pentlandite-pyrite at the edges of the deposit. A higher-grade central core containing >1.5% Ni can reach a thickness of 65m. The central core is enveloped by disseminated sulphides of lower grade (0.5-1.5% Ni). Sulphide mineralisation bifurcates at the northern end and eventually terminates, due to steeply dipping cross cutting pegmatite dykes that are interpreted to have been intruded along brittle fault structures.
The ODN orebody is located 1,000-1,075m below surface and is interpreted to be the northern continuation of Odysseus mineralisation. The ODN and ODS deposits appear to be bifurcated and offset by northeast striking faults now sealed with pegmatite. The entire ODN orebody is dextrally offset ~60m in relation to Odysseus and the dip is rotated 50° counter-clockwise (now near horizontal).