Although komatiite has been defined as an ultramafic volcanic rock characterized by spinifex texture, there is a growing recognition that similar textures can also form in high-level dykes and sills. Here, we report the results of a petrological and geochemical investigation of a 5m thick komatiite sill in Dundonald Township, Ontario, Canada. This unit forms part of a series of komatiites and komatiitic basalts, some of which clearly intruded unconsolidated sediments. The komatiite sill is differentiated into a spinifex-textured upper part and an olivine cumulate lower part. Features characteristic of the upper sections of lava flows, such as volcanic breccia and a thick glassy chilled margin, are absent and, instead, the upper margin of the sill is marked by a layer of relatively large (1–5mm) solid, polyhedral olivine grains that grades downwards over a distance of only 2 cm into unusually large, centimetre-sized, skeletal hopper olivine grains. This is underlain by a 1m thick zone of platy

Abstract: The Kambalda nickel sulphide deposits are located in the 2700Ma Eastern

The nickel sulfide deposits of Western Australia consist of many intrusive dunitc-associated and volcanic peridotite-associated deposits of late Archcan age, a few small gabbroid-associated deposits of Archcan or Proterozoic age, and some other rare types. Some 96 percent of the nickel metal resource is contained in deposits concentrated in the curvilinear, folded and strike-faulted supracrustal belts of the eastern Yilgarn Block. Very magnesian ultramafic rocks and sulfidic metasediments abound in these belts, which have commonly been metamorphosed to greenschist or lower amphibolite facies conditions. Major strike faults probably influenced the nature and distribution of volcanism and sedimentation, later tectonometamorphic styles, and nickel mineralization. Intrusive dunitc-associated deposits occur in semiconcordant lenses of peridotitc to olivinite composition and of komatiitic affinities, which are restricted to curvilinear zones 100 km or more long. Nickel sulfides occur centrally or marginally in the thickest part of the lens. Low-grade (< 1 % Ni) sulfides are abundant and enclose smaller bodies of higher grade disseminated and massive or breccia ores, which may be in part tectonically displaced. The Ni content of