Steeply dipping massive textured nickel-sulphide mineralisation associated with metamorphosed ultramafic lava flows.

In the Cliffs section, the Cliffs Ultramafic Unit has a total thickness of up to *420 m: the upper unit, which comprises a series of stacked spinifex-textured flows, can be up to *200 m thick, and the peridotite base generally attains similar thicknesses. The discrete flows making up the upper unit display a typical range of spinifex textures, from fine random to coarse plate, which correlate with the A–B komatiite zonation described by Pyke et al. (1973). Conversely, the lower cumulate unit contains both orthocumulate and mesocumulate textural zones, and is dominated by mesocumulate texture. Pseudomorphed olivine occurs as both equant and elongated crystals and is typically in the range 2–10 mm long, although in areas the long axis of crystals can reach 2 cm. Bimodal crystal size distributions occur in places and have a population of elongated crystals up to 2 cm-long set in a matrix of finer (2– 10 mm) former olivine grains. Typically, the elongated crystals display a shape-preferred orientation, with their long axis parallel to the flow top. Thin (up to *10 cm thick) horizons of skeletal and hopper olivine crystals, ranging in size from 8 mm to 1.5 cm, occur sporadically in the lower peridotite unit. The coarse mesocumulate grades into a finer orthocumulate along strike from Cliffs as the Cliffs Ultramafic Unit thins to *180 m northward.

The Cliffs Ni deposit is located along the basal contact of the Cliffs Ultramafic Unit (lower peridotite), and is developed along a strike extent of *800 m. Typically, the mineralised zone comprises a basal massive Fe–Ni–Cu sulfide zone, overlain by matrix and stringer sulfide, and a final upper zone of disseminated sulfides. The basal massive sulfide horizon varies in thickness along its strike length from *5 cm to *9 m (Rosengren 2004). Local thickenings are inferred to represent structural remobilisation related to heterogeneous strain. The primary sulfide assemblage comprises pyrrhotite pentlandite–chalcopyrite–pyrite (Grguric et al. 2006).

The Cliffs Nickel Mine extracts a narrow near vertical nickel vein using a mining method of underhand bench stoping with paste backfill.

Nickel sulphide ore is mined at the Cliffs and processed through a concentrator and dryer at Leinster.