||Greenstone in northern Minnesota is somewhat younger
than gneiss. Greenstone is a weakly metamorphosed (altered) basalt that
is, as its name suggests, greenish to gray. This type of rock formed about
2.7 billion years ago when the area that is now northern Minnesota was part
of a volcanic island arc, much like the islands of Japan are today. Greenstone
and other associated volcanic and related rocks have in the past been prospected
for deposits of economic metals such as gold, copper, zinc, lead, and iron.
Iron mines at Ely and Soudan are now closed. As yet, no other significant
metal deposits have been found.