Komatiites and Ni-Cu-PGE Deposits

Mining & Minerals - Evaluations / Other Metals

Gavryliv reports that (pronounced ko-ma-tee-ites) are an unusual and very ancient volcanic rock. They are interesting to geologists for their unique “spinifex” texture, their exceptional chemical composition and the high-temperature conditions of their formation. They are also host to a significant portion of world nickel production. With an eruption temperature of nearly 1600 °C, komatiitic magma was 400-500 °C hotter than that of the basaltic magma erupting from the active volcanoes today.

It is believed that the predominance of the komatiites in the Archean (4.0 – 2.5 billion years ago) with minor occurrences in Proterozoic (2.5 – 0.54 billion years ago) and a few in Phanerozoic (0.54 billion years ago to present) is associated with the progressive cooling of the Earth’s mantle. However, komatiite-hosted nickel deposits are found only in the Archean complexes of the Precambrian cratons: the Barberton greenstone belt (South Africa), the Abitibi Belt (Canada) and Kambalda (Western Australia). This article takes attempts to explain komatiite formation and characteristics, and to elucidate the economic importance of these rocks.

Komatiites are mantle-derived volcanic rocks typically occurring within Archean and Proterozoic sequences of metamorphosed volcanic and sedimentary rocks called greenstone belts. Accordingly, the term ‘komatiite’ is applied to rocks composed of >90% of mafic minerals (rich in magnesium and/or iron, like olivine, pyroxene and biotite), which are often present as large skeletal or dendritic crystals producing a texture called ‘spinifex’ (spinifex texture). It is thought that the rapid cooling of komatiitic magma is responsible for development of this special texture.

Komatiites account for about 10-14% of the world’s annual nickel production. After the discovery of the Kambalda deposit in the southern part of the Norseman–Wiluna belt, Western Australia komatiite-hosted sulfide deposits were divided into two principal types: Type 1, ores occur as massive sulfides at or near the basal contacts of the host komatiites in the channels created by the lava flows while Type 2 occur as disseminated sulfides in the olivine-rich komatiitic bodies called “cumulates”.

Komatiites are exclusive ancient rock types which exhibit a unique spinifex texture. The Ni-Cu-PGE mineralisation generally occurs as massive sulfide ore at the floor of the komatiitic flow or as disseminated sulfides through olivine-rich layers. It is worth noting that the critical features whether the mineralisation will occur are the flow rate and more importantly the presence of a sulfur source in the underlying volcanic or sedimentary rocks.

For additional information on a South African deposit, see (here).

and for nickel in general, see (here).