Komatiitic ultramafic rocks are common in Archean greenstone-granite terranes. The Eastern Goldfields Superterrane (EGST) of the Yilgarn Craton, Western Australia, is of particular interest as it hosts many nickel sulphide deposits in abundant thick cumulate-rich komatiite packages within the greenstone belts. Komatiites magmas are characterised by high MgO contents (> 18 wt. %) and show well-documented flow profiles of olivine cumulates overlain by spinifex-textures and flow top facies (e.g., Barnes 2006). Komatiite associated massive nickel sulphides accumulate in narrow channelised deposits at the base of the cumulate package, along which distinct mineralisation profiles and ore body morphologies, including open contact and pinch out geometries with the underlying basalt develop through thermo-mechanical erosion (Lesher et al., 2002). Whilst it is critical to understand the spatial distribution and morphology of volcanological features to delineate ore bodies within the komatiite package, challenges frequently arise from post-volcanic deformation, metamorphism and alteration of the host komatiitic sequences. This is especially important when trying to distinguish multiple komatiite flows from structural repetitions, as hypothesised for areas of the Yilgarn Craton (e.g., Swager and Griffin, 1990). Here we examined the structural history of the recently discovered Cassini Ni deposit (Mincor Resources), located in the southern apex of the Widgiemooltha Dome of the Kalgoorlie Terrane, to unravel the deformation overprint of komatiitic volcanic facies and associated nickel sulphide deposits. New lithological classifications were derived from company whole rock geochemical database and coupled with structural logging and 3D analysis of the datasets to resolve subsequent overprinting regional Neoarchean deformation. Nickel tenor analysis revealed two possible magmatic flow channels along the basal contact of the Mount Morgan Komatiite. The earliest deformation generated a foliation at low angles to lithological contacts as well as overthrusts noted above massive and net textured sulphides. These structures and the Cassini ore body were passively refolded during regional D2 ENE-WSW deformation. Subsequent oblique slip deformation is localised along the eastern limb of the Widgiemooltha anticline. The best preservation of the classic mineralization profile is documented in parasitic F2 synclines on the western limb of the Widgiemooltha anticline. The F2 axial trend is subparallel to the highest tenor trend, providing critical guidance for understanding modification of the primary channel morphology. The results of this study have important implications for improving prediction of magmatic nickel sulphide deposits in poly-deformed terranes.

Reference(s)

Barnes SJ, 2006, Komatiites: Petrology, Volcanology, Metamorphism, and Geochemistry in Nickel Deposits of the Yilgarn Craton: Geology, Geochemistry, and Geophysics Applied to Exploration. Society of Economic Geologists Special Publication No. 13, p. 13–49. 

Lesher C, Keays RR, Cabri L, 2002 Komatiite-associated Ni-Cu-PGE deposits: Geology, mineralogy, geochemistry and genesis. Canadian Institute of Mining, Metallurgy and Petroleum, v. 54, p. 579-618. 

Swager, C., and Griffin, T. J., 1990, An early thrust duplex in the Kalgoorlie-Kambalda greenstone belt, eastern Goldfields Province, Western Australia: Precambrian Research, v. 48, p. 63-73