Komatiite avec un facies Spinifex bien marqué dans un cumulat magmatique daté à -2.7 Ga. (Upper Komatiitic Unit ; Pyke Hill proche de Potter Mine, Ontario, Canada).
Komatiite sill in the Precambrian of Ontario, Canada. (this outcrop is bleached to prevent growth of vegetation; stratigraphic up is toward the top; this is a NO HAMMER locality) 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. Within each lava flow at Pyke Hill, the silica content increases upward. The peridotites (= lower portions of each flow) are 37 to 45% silica, while the spinifex-textured rocks (= upper portions of each flow) 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. At this particular site, komatiite sills having a coarsely-crystalline spinifex texture have intruded a relatively thick, cumulate-textured komatiite lava flow. The sills are congruent to subcongruent with lava flow bedding. The chill margins of the sills are finely-crystalline, whereas olivine crystal sizes in the lava flow itself become coarser next to the flow-sill contact. The sills are interpreted as having been intruded when the lava flow was still hot and semiconsolidated (see Houlé et al., 2010). Stratigraphy: Upper Komatiitic Unit, upper Kidd-Munro Assemblage, Abitibi Greenstone Belt, lower Neoarchean, 2.711-2.717 Ga Locality: Pyke Hill outcrop - hillside exposures on the northwestern part of Pyke Hill, east of the Potter Mine, north of Route 101, east-northeast of Matheson & south of the western end of Lake Abitibi & ~83 kilometers east of the city of Timmins, Munro Township, southern Cochrane District, eastern Ontario, southeastern Canada (vicinity of 48° 35' 55.78" North latitude, 80° 12' 00.05" West longitude) (= locality B (“Spinifex veins penetrating underlying peridotite”) on the northwestern part of Pyke Hill of Fyon & Green, 1991 - Geology and ore deposits of the Timmins District, Ontario (field trip 6), Geological Survey of Canada Open File 2161, p. 29 (figure N13).) (= stop 2.1C of Houlé et al., 2010 - Stratigraphy and physical volcanology of komatiites and associated Ni-Cu-(PGE) mineralization in the western Abitibi Greenstone Belt, Timmins area, Ontario: a field trip for the 11th International Platinum Symposium. Ontario Geological Survey Open File Report 6255, p. 36 (figure 17) & p. 37.)