Pioneer Nickel Ltd - Nickel Sulphide Ore Genesis Model

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Nickel Sulphide Ore Genesis Model

Most ultramafic rocks from the Eastern Goldfields of Western Australia is interpreted as the product of a cataclysmic eruption of komatiite magma with very high effusion rates.

Olivine cumulate rocks are very highly magnesian ultramafic rocks ("komatiites") with tightly packed crystals formed by the build up of, almost exclusively, olivine crystals deposited from turbulently flowing rivers of lava, possibly through lava tubes. Under certain circumstances nickel sulphide deposits formed at the basal contact of these deposits.

With time, an olivine crystal pile will build up, crystallizing from a continually replenishing turbulent lava flow. Breakouts, thin flows and flow tops are marked by distinctive "spinifex textured" rocks. As the influx of fresh lava eventually waned, lava ponded and crystal fractionation resulted in the formation of olivine-pyroxene and plagioclase-pyroxene cumulates and gabbro at the top of the lava pile. These latter rocks are less magnesian-rich and by detailed study the geologist can map each of these "facies" building up a predictive map of where the prospective basal olivine cumulate contact is.

Komatiite-hosted nickel sulphide deposits such as those mined at Kambalda, Widgiemooltha, Spargoville, Silver Swan and Cosmos are typically hosted by relatively thin (less than 100 metres thick) komatiite lava flows. Sulphides are usually massive at the base of the flow, with or occurring as intercumulus inclusions (disseminated or matrix sulphides) higher in the stratigraphy.

Massive sulphides accumulate at the base of the komatiite sequence through being immiscible with the flowing lava. Even when solidified, massive sulphides remain exceptionally mobile, and often accumulate within structurally controlled, thermally eroded channels, or are remobilised into the surrounding country rock.

Ore genesis and exploration models therefore emphasise:

Confinement of the Nickel Sulphide ore shoots to physical embayments into the footwall at the contact position with the overlying komatiite ultramafic.
Restriction of thick, high-MgO ultramafic rocks to stratigraphically higher positions within the overall volcanic package.
Paucity or absence (through thermal erosion) of sedimentary rocks at the footwall contact.
Massive ore can become remobilised from the basal contact, thus exploration needs to include the immediately adjacent rocks, particularly within the footwall.

The mineralised embayments, or troughs, are generally elongate, linear features up to kilometres long and tens of metres deep. Stratigraphic and volcanic studies interpret the troughs as syn-volcanic features modified during post-volcanic deformation.

Gross ultramafic whole rock nickel distribution is used to identify enriched volcanic channels, providing a vector towards mineralisation. Values above 1,500 ppm Ni act to discriminate between channel (mineralised) and non-channel komatiites. The 3,000 ppm Ni threshold value delineates potentially high tenor ore environments.

Pyrrhotite is the dominant sulphide, and is the last to form after sulphur, nickel and copper have exolved to form pyrite, pentlandite and chalcopyrite respectively. Hence pyrrhotite often forms the matrix to the other sulphide minerals.

Nickel Sulphide Ore Emplacement Model

Figure 1: Cross-sectional model of the Nickel Sulphide ore environment. Interpreted from drilling data.