File:Peperite (Middle Tholeiitic Unit, Kidd-Munro Assemblage, Neoarchean, 2.711-2.719 Ga; just southwest of the Potter Mine, east of Timmins, Ontario, Canada) 5 (46970624585).jpg

Peperite in the Precambrian of Ontario, Canada.

These unusual-looking rocks are part of the Late Archean-aged 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.

This outcrop consists of an igneous dike (= just off the margins of the photo), and abundant breccia clasts. The dike intruded wet sediments and brecciated itself, forming peperite. According to the British Geological Survey, peprite is a "lithified, fluidized mix of hyaloclastite and sediments formed by the disruption & rapid chilling of magma when intruded into, or flows over, wet sediments." (see Gillespie & Styles, 1999). It's very rare to get well-preserved peperite in the Archean.

Description from Houlé et al. (2010) (the dikes are referred to as sills):

[B]asaltic and komatiitic sills [that] are emplaced into the volcaniclastic succession are interpreted by Gibson and Gamble (2000) as high-level synvolcanic intrusions. Evidence for this interpretation includes: 1) their fractured and autobrecciated upper and lower contacts with massive volcaniclastic material, massive sulfide and/or argillite injected along fractures that penetrated the massive sill interior; 2) locally chilled and sharp upper and lower contacts; 3) the development of hyaloclastite along chilled and perlitic-textured sill contacts and the mixing of this hyaloclastite with enclosing argillaceous mudstones and sulfide to form peperite; and 4) a basaltic composition that is identical to the volcaniclastic rocks.

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

Locality: outcrop next to a little dirt road that is just southwest 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' 53.18" North latitude, 80° 12' 46.05" West longitude) (= stop 2.2D2 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. 34 (figure 16) & p. 39.)

Additional reference cited: