History & Geology
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The Birimian Greenstone Belts of West Africa
June 2013 by Chris
A few years ago I had a tremendous opportunity to visit Australia and hunt for gold there. The geology of the gold deposits there was somewhat different from what I was used to here in the US. I learned a lot about greenstone belt gold deposits while prospecting them in person, and it was an amazing trip. I heartily recommend Australia to any prospectors who have the money and the desire to visit.
GeologyAfrica actually has a number of productive greenstone gold belts, but the Birimian Belts of West Africa are the best known and most productive on the continent. These types of deposits form from the collision of tectonic plates. The heat and pressure of the collision of these plates upon volcanic rocks—often ocean floor basalts—creates the hot, gold-bearing fluids that form the deposits. The main goldfields being explored in western Africa lie within the Proterozoic rocks of the Man Shield, which is part of the West African Craton, a body of very old rocks composed of granitic gneisses and greenstone belts.
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. Here gold is associated in the margins of belts of metamorphosed volcanic rocks commonly referred to as greenstone. Almost without exception, the major gold deposits of Ghana and the rest of West Africa lie at or close to the contact margins of these greenstone belts where they meet up with other rocks like schist, and these structural contacts are often strongly deformed and associated with shearing forces. These greenstone belts are associated with zones of extensive shearing and faulting expressed by parallel, steeply dipping, deep fault systems, which are developed at the contact between the meta-volcanic and meta-sedimentary rocks (schist, slate and similar materials).
Gold mineralization is also associated with sheeted quartz vein swarms and stockwork zones found within granitic intrusions. These small- to medium-sized granitic intrusions are present within the greenstone belts. Because the mineralized contact zones continue on for long lengths, the gold deposits continue to trend periodically along the contact quite a distance. Mineralized zones can even continue across country borders as the minerals are not limited by lines on a map.
Because of its great age, the area has generally been worn down by the long effect of weathering and is fairly flat. While some parts are heavily vegetated, the majority of the gold-bearing areas are fairly arid and desert-like. A majority of these goldfields are located in what would be considered dry savannah or desert regions. The area is dry because it is situated along the southern margin of the great Sahara desert, one of the driest locations on Earth.
The West African craton goldfields have quite a bit in common with the ones I explored in West Australia. They are both of great age and caused by the same types of geologic forces. Both are important gold producers. Perhaps the biggest difference is the presence of large bodies of ironstone in Australia, which are not present in West Africa. In Australia, ironstone is present nearly everywhere in the gold-bearing areas I visited. In West Africa, bodies of manganese and iron-rich rock are found in place of the ironstone. I don’t know if these rocks will be noisy and difficult to work with a metal detector. The ironstone certainly was a problem, but manganese is usually not nearly as bad. I will be finding out soon!
While I stated that the amount of political unrest is diminishing in this region, there have been recent civil wars in Sierra Leone, Liberia and Côte d’Ivoire. Mali was recently invaded by French armed forces to control terrorists in the northern part of the country. In addition, there have also been some other cases of regional upheavals and political protests. So, while things are greatly improved, the area is not completely stable. While some countries are safe enough that they are regularly visited by tourists, others are not so safe.
I know a number of readers will be wondering about the potential opportunities for individual prospectors to hunt gold in this part of the world. I wish I could say that West Africa is an easy region to visit, but as far as I know, there are no tours or invitations to prospectors coming from the outside. Big mining companies get special invites from the government. Getting around is not easy. Not every town has a store for supplies and fuel. The roads connecting up the countryside are dirt, not asphalt. Diseases like malaria, yellow fever and even polio are common here. There are very real dangers out in the goldfields and sometimes people without assets will resent outsiders coming in to take their gold.
The group I am going with represents a special opportunity, and I am not going in just as a tourist out hunting for gold. Even so, while a part of me is very strongly looking forward to an amazing adventure, the other part of my brain is telling me that I am crazy for even attempting this. There are also language barriers to be overcome, as French is the primary language for most of the countries in this area. A couple of them are English speaking, but out in the goldfields even English and French are not sufficient—many people speak only the local tribal languages.
I had wished my trip to Australia was perhaps 15 years earlier when so many of the nugget patches had yet to be found. This trip to West Africa is in some ways the fulfillment of that—a raw and mostly unexplored part of the world.
As I write this I am only a few days from my departure. When I get back, I will share my photos and experiences. So although you may not be able to travel to western Africa, you can still get a flavor of what I hope will be a very unique prospecting adventure.
© ICMJ's Prospecting and Mining Journal, CMJ Inc.