A Tale of Greenstone Gold

There are a number of different types of geologic settings that lead to the formation of gold nuggets. Some are rare, and some occur at many places around the world. One of the more common is associated with greenstone belts. 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). 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. Greenstone belts are zones of parallel rocks that include these meta-basalt rocks and a variety of associated formations.

These four greenstones are from gold deposits located all around the world, and show how common this type of deposit is. 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.

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. Even in California and Arizona, rocks associated with meta-basalt greenstone have produced gold. This is not just an exercise in theoretical geology, some of the very best nugget patches ever found occur at the contacts of these meta-basalt greenstones and adjoining rocks, so this is a kind of rock worth learning to recognize. The contacts of these rocks form long lines of mineralization creating a series of patches and deposits which can be many miles long.

In West Australia, the main greenstone belts include:

• Southern Cross Belt
• Norseman-Wiluna Belt|
• Duketon Belt
• Gullewa Belt

All of these have been highly productive for prospectors armed with metal detectors. Gold deposits, both open pit commercial in nature as well as alluvial nuggets are distributed along these belts. They can be traced on geologic maps for long distances. Other small bits and pieces of un-named greenstone belts running parallel to these and the smaller stringers can have gold associated with them as well.

Steve Herschbach explores a greenstone patch in Western Australia. The surface is well weathered and does not look like fresh greenstone, but where the scrape made here gets deep enough, fresh greenstone was exposed. Many of the rocks are greenstone coated with a surface of rust red iron oxides. 

Recently, I had an opportunity to see some of the greenstone belt gold deposits of West Africa. This area has become the target for gold exploration by large mining companies and a tremendous number of finds have been made in recent years, and detector operators have dug their share of gold as well. The goldfields of western Africa cut across national boundaries, so many different small nations are involved in this new gold rush. The most productive gold bearing zone in this area to date has been the Ashanti belt in Ghana which has produced gold for centuries and been actively mined by European operators since the 1800s. However, other greenstone deposits have not been as well explored and many new discoveries are expected in future years. While I was there I got to meet prospectors who were a part of this new gold rush and with their detectors and found enough gold to radically change their lives and bring new wealth to themselves and their families. I even detected my first African nugget.

So the next time you are out prospecting, keep an eye out for this very productive rock and look for the gold associated with it.