Greenstone

Greenstone is a term used in geology to describe metamorphosed volcanic and sedimentary rocks that are commonly preserved in ancient crust. The rocks typically acquire a greenish hue from minerals such as chlorite, actinolite, epidote, and serpentine. The phrase is often applied to large, coherent sequences known as greenstone belts, which are among the oldest exposed terrains on Earth.

Greenstone belts formed in the early Earth, roughly between 3.0 and 2.5 billion years ago, in volcanic arc and submarine volcanic–sedimentary settings. They consist of metamorphosed basalts and andesites, interlayered with sediments, and later recrystallized to schists, phyllites, and other foliated rocks. The belts are commonly bounded by granitic intrusions and older crust, reflecting a history of arc magmatism, crustal growth, deformation, and regional metamorphism.

Distribution and significance: Major greenstone belts occur in the Canadian Shield (for example the Abitibi belt), in South Africa’s Barberton region, in Australia’s Yilgarn and Pilbara blocks, and in parts of West Africa and South America. They are economically important because they host rich mineral deposits, particularly gold, as well as nickel, copper, zinc, and platinum-group elements, often in association with sulfide mineralization within metavolcanic rocks.