Summary:
Mt Keith
Disseminated textured magmatic nickel-sulphide mineralisation associated with a metamorphosed ultramafic intrusion.
The stratigraphy in the Mt Keith region is composed of ultramafic sequences (olivine cumulates and thin spinifex textured flows), basalts, volcanic and volcanoclastic rocks and sedimentary rocks The ultramafic sequence at Mt Keith is lenticular It faces W, dips 80- 85, and has undergone low- to mid-greenschist metamorphism The major komatiitic rock types include sulphide-bearing olivine mesocumulate, olivine orthocumulate, talc-carbonate altered orthocumulates and a porphyry olivine rock The non sulphide mineralogy consists of hydrated Mg silicates (Iizardite, antigorite, chrysotile, talc), Mg oxy carboxyl hydroxides (brucite, stichtite, pyroaurite, iowaite), spinels (chromite, magnetite), carbonates (magnesite, dolomite), tremolite-actinolite and chlorite The disseminated sulphide mineralogy is dominated by pentlandite, with minor millerite, viola rite and pyrrhotite Trace amounts of pyrite, chalcopyrite, heazlewoodite, gersdorffite, polydymite, tochilinite, sphalerite and exsolved Co sulphides are also present.
Leinster
Steeply dipping disseminated and massive textured nickel-sulphide mineralisation associated with metamorphosed ultramafic lava flows and intrusions.
BHP Billiton’s Leinster Nickel Operation (LNO) is located 600 km north-east of Perth in the Eastern Goldfields Region of Western Australia. The LNO deposits are situated within the Wiluna Greenstone Belt and the orebodies are hosted in Komatite Ultramafic lava flows.
Cliffs
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).