File:Peridotite & serpentinite (Upper Komatiitic Unit, Kidd-Munro Assemblage, Neoarchean, 2.711-2.717 Ga; southwest of the Potter Mine, east of Timmins, Ontario, Canada) 9 (47104399434).jpg

Peridotite & serpentinite in the Precambrian of Ontario, Canada.

Brownish = peridotite (= cumulate-textured portion of a thick komatiite lava flow) Grayish = serpentinite

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 komatiite "lava lake" next to the Potter Mine in Ontario, Canada. This is near the world-famous Pyke Hill locality, which has numerous, thin komatiite lava flows. The rocks in the Potter Mine-Pyke Hill area 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 rocks at this particular site were originally interpreted as part of a relatively thick komatiite lava lake, the last feature of a volcanic eruption. A newer interpretation says that this is a series of thick komatiite sheet flows (at least 6) in a paleo-depression. A thin lava lake may have originally capped the succession.

This is an exposure of partially serpentinized komatiite. The brownish and grayish-brown colored rocks are an olivine cumulate-textured portion of a relatively thick komatiite lava flow - referred to as "peridotite" in the literature. The gray and light gray areas are serpentinite, a low- to high-grade metamorphic rock composed of serpentine (= magnesium hydroxy-silicate, Mg3Si2O5(OH)4). Serpentinite forms by metamorphism of olivine-rich igneous rocks (e.g., peridotite, dunite, komatiite), in the presence of water.

Unweathered serpentinite has a mottled greenish color, often has the look & feel of hard candle wax, and ranges in texture from crystalline to “foliated”. Many serpentinites have a foliated look to them, but it’s really not due to an planar alignment of crystals. The appearance of “foliated” serpentinites is really the result of extensive development of slickenlined surfaces. The original igneous textures may still be evident.

Many serpentinites have a component of magnetite (= iron oxide, Fe3O4) that is usually significant enough to feel a slight tug when a magnet is placed next to the rock. Some serpentinites have "veins" of white asbestos (= chrysotile serpentine).

Description of site from Houlé et al. (2010):

Toward the upper part, the "Lava Lake" exposure is characterized by intense serpentinization. An irregular network of hierarchized bands of serpentine divides parts of the dunite/peridotite into many irregular and randomly distributed remnants similar to those observed within a serpentinized olivine crystal in thin section. [that refers to mesh texture]

Stratigraphy: Upper Komatiitic Unit, Kidd-Munro Assemblage, Abitibi Greenstone Belt, lower Neoarchean, 2.711-2.717 Ga