Even in California and Arizona, rocks associated with meta-basalt greenstone have produced gold. Most of the gold of Western Australia is related to these types of deposits, but they have also produced substantial amounts of gold in Canada, Brazil, as well as both east and west Africa. The piece on the lower left is from Australia, the upper left is from West Africa, the lower right is from Arizona, and the upper right from California in the US. These four greenstones are from gold deposits located all around the world, and show how common this type of deposit is.
Greenstone belts are zones of parallel rocks that include these meta-basalt rocks and a variety of associated formations. This gives a dull green colored rock which, by the same forces of heat and pressure is very commonly associated with gold all over the world. Not every green colored rock is a greenstone, and what geologists are normally talking about with greenstone is a rock that once was black basalt on the ocean floor, but which has been subjected to the forces of heat and pressure that has changed the rock (metamorphic forces geologists call it). One of the more common is associated with greenstone belts. Some are rare, and some occur at many places around the world. There are a number of different types of geologic settings that lead to the formation of gold nuggets.
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