File:Metakomatiite lava flows (Upper Komatiitic Unit, Kidd-Munro Assemblage, Neoarchean, 2.711-2.719 Ga; Pyke Hill, near the Potter Mine, east of Timmins, Ontario, Canada) 28 (32884292027).jpg

Metakomatiite (serpentinized) in the Precambrian of Ontario, Canada. (this outcrop is bleached to prevent growth of vegetation; stratigraphic up is to the left; this is a NO HAMMER locality)

Units from left to right (4): - unit B3 - knobby peridotite, with knobs of clinopyroxene, surrounded by hydrated/serpentinized olivine - unit B4 - basal peridotite at base of komatiite lava flow - unit A1 - flow top breccia of komatiite lava flow - unit A2 - spinifex zone

Komatiites are very rare, magnesium-rich, extrusive, ultramafic igneous rocks. They are named after the Komati River Valley in South Africa, the type locality. Komatiite is an exceedingly rare type of lava. No volcano on Earth erupts this material today. Komatiites are essentially restricted to the Archean (4.55 to 2.5 billion years ago). Experimental evidence has shown that komatiite lavas, when originally erupted, were considerably hotter (~1600º C) than any modern lava type on Earth. This indicates that Earth’s mantle was much hotter than now. Other geologic evidence also indicates that early Earth’s heat flux was much higher than today’s.

Komatiite lava had a very low viscosity - it could flow like an ultradense gas. This property permitted the solidification of some individual lava flows that are only 1 cm thick.

The classic texture of komatiites is spinifex texture, named after clumps of long, spiky (& painful!) grasses. Komatiites with spinifex texture have short to long blades or plates of olivine mixed with smaller-scale blades of pyroxene.

All Archean komatiites are metamorphosed - the original igneous mineralogy (olivine, pyroxene, minor chromite, etc.) is gone to mostly gone. Such rocks are best termed metakomatiites, but the prefix “meta-” is usually not specified in writing.

Komatiites have economic significance, as many are closely associated with copper-nickel minerals (chalcopyrite & pentlandite), plus minor platinum-group elements, arsenides, bismuthides, and maybe a little gold and silver. Komatiites are a world-class source of nickel in Canada and Western Australia.

The outcrop seen here is part of a stack of tectonically-tilted, nearly vertical, komatiite lava flows at Pyke Hill, Ontario, Canada. The original olivine spinifex texture is serpentinized for the most part. Pyroxene spinifex consists of small, felted needles. Cumulate-textured units are also present in these lava flows, plus knobby peridotite units and brecciated or cooling-cracked flow tops.

Pyke Hill is a world-famous locality for komatiites. The rocks there are part of the Kidd-Munro Assemblage, which consists of ultramafic and mafic volcanic rocks intruded by mafic to ultramafic dikes and sill-like bodies. Minor felsic volcanic rocks are also present. Volcanogenic massive sulfide deposits occur in the Kidd-Munro Assemblage - they have been mined at the Texas Gulf Mine and the Potter Mine.

The peridotites at Pyke Hill are 37 to 45% silica. Spinifex-textured rocks here are 42 to 45% silica. The chromium (Cr) content ranges from 1500 to 5000 parts per million in the peridotite and spinifex-textured rocks. Nickel (Ni) content ranges from 400 to 2000 parts per million.

Stratigraphy: Upper Komatiitic Unit, Kidd-Munro Assemblage, Abitibi Greenstone Belt, lower Neoarchean, 2.711-2.719 Ga