Gabbro refers to a large group of dark, often coarse-grained, mafic intrusive igneous rocks chemically equivalent to basalt. It forms when molten magma is trapped beneath the Earth's surface and slowly cools into a holocrystalline mass. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges.
Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.
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