Greenstone

Greenstone is a broad field term for any compact, dark green, low-grade metamorphosed mafic igneous rock, typically altered basalt or related volcanic rock. Its green color comes from secondary minerals such as chlorite, epidote, and actinolite that grow during low-grade metamorphism or hydrothermal alteration.

The term is most famous in the context of Archean 'greenstone belts,' ancient volcanic-sedimentary sequences that host major gold and base-metal deposits. Note that in New Zealand, 'greenstone' is also used for nephrite jade (pounamu), a separate meaning.

Greenstone in the geological sense is widespread in the world's oldest cratons and shield regions.

Greenstone forms when basaltic and other mafic volcanic rocks are subjected to low-grade metamorphism (greenschist facies) or seafloor hydrothermal alteration. The original igneous minerals, such as olivine, pyroxene, and calcic plagioclase, are converted into green secondary minerals including chlorite, epidote, actinolite, and albite.

Much greenstone occurs in Archean and Proterozoic greenstone belts, elongate tracts of metamorphosed volcanic and sedimentary rock preserved within ancient granitic cratons. These belts formed in early oceanic and arc settings billions of years ago and are economically vital as hosts of gold and base metals. Famous examples include the Abitibi belt in Canada and the Barberton belt in South Africa.

Greenstone is recognized by its dull green to greenish-gray color, fine to medium grain, and generally massive to weakly foliated texture. The green tint comes from chlorite, epidote, and actinolite; the rock is often compact and tough.

Because 'greenstone' is a loose field term, precise identification of the protolith may require thin sections. Distinguish geological greenstone from New Zealand 'greenstone' (nephrite jade), which is much harder, tougher, and translucent. Serpentinite is softer and waxier, while greenschist proper shows clearer foliation. Finding it as altered basalt within an ancient volcanic terrain supports the identification.

Greenstone belts are among the world's most important sources of gold, as well as copper, zinc, and other metals, making greenstone hugely significant economically. The rock itself is used locally as crushed aggregate and building stone.

In prehistory, tough greenstone was prized for polished stone axes and tools across Europe and elsewhere. In New Zealand the same word refers to nephrite jade, a treasured carving stone in Maori culture. As a geological material proper, greenstone has little metaphysical following, but its association with gold gives it great practical importance.