Greenstone belt
From Wikipedia, the free encyclopedia
- For other meanings, see Greenstone (disambiguation).
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The belts have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes. Archaean greenstones are found in eastern and northern Canada, western Australia, south and eastern Africa and Madagascar, west Africa and Brazil, and northern Scandinavia and the Kola Peninsula. Proterozoic greenstones occur in west Africa, northern Madagascar, northern Canada and northern Scandinavia.
The name comes from the green hue imparted by the chlorite minerals within the rocks. The belts often contain ore deposits of gold, silver, copper, zinc and lead.
One of the best known greenstone belts in the world is the South African Barberton Greenstone belt, where gold was first discovered in South Africa. The Barberton Greenstone belt was first uniquely identified by Prof Annhauser at the University of the Witwatersrand, Johannesburg, South Africa. His work in mapping and detailing the characteristics of the Barberton Greenstone belt has been used a primer for other greenstone belts around the world.
He noted the existence of pillow lavas, indicating a lava being rapidly cooled in water, as well as the spinifex textures created by crystals formed under rapidly cooling environments, namely water.
[edit] References
- Maarten J de Wit and Lewis D Ashwal (1997) Greenstone Belts, Clarendon Press ISBN 0-19-854056-6 Excerpts and maps
- Lowe, D. R. (1994) Accretionary history of the Archean Barberton Greenstone Belt (3.55-3.22 Ga), southern Africa Geology. 1994 Dec;22(12):1099-102. Abstract
