Abstract

Considerable progress has been made in the radiometric dating of Precambrian iron-formations, but the age of relatively few is known precisely. Early Precambrian or Archean deposits, older than 2,600 m. y., are closely associated with greenstone belts. Many, if not all, of these older iron-formations on the Canadian Shield were formed in the interval from 2,750 to 2,700 m.y. ago. Banded iron-formations in Western Australia may date back to 3,000 m. y. and some in South Africa, Venezuela, and the USSR may be well over 3,000 m. y. old.The greatest development of banded iron-formations occurred between 2,600 and 1,800 m. y. ago in the Middle Precambrian or Lower Proterozoic (Aphebian of the Geological Survey of Canada). The huge iron-formation of the Hamersley Range in Australia is dated between 2,200 and 2,000 m. y. A similar age is a reasonable estimate for Krivoy Rog, Ukrainian Shield. Rb-Sr isochron ages on metasedimentary rocks associated with iron-formations in the Lake Superior region and in eastern Canada range from 1,890 m. y. (Sokoman Iron Formation, Labrador Trough, Quebec) to 1,700 m. y. (Gunflint Iron Formation, Thunder Bay district, Ontario). The Rb-Sr ages have been considered to represent the time of deposition, but are here interpreted as secondary or metamorphic ages. In this interpretation a minimum age of 1,900 m. y. is indicated, but the available data do not preclude the possibility that the Middle Precambrian iron-formations on the Canadian Shield are approximately 2,100 m. y. old.The iron-formation of the Rapitan Group in the Snake River area, northwestern Canada has not been dated isotopically, but has been correlated with the Windermere System of Late Precambrian age.

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