Archaean-age greenstone belts in particular tend to host rich gold deposits. One example is Canada’s Abitibi greenstone belt, which is home to a slew of gold mines.
Unsurprisingly, many exploration and mining companies are interested in operating on greenstone belts. Below we run through how these geological formations are created, where major greenstone belts can be found and which greenstone belts could have future potential. Read on to learn more.
How are greenstone belts formed?
Greenstone belts can be described as “exposed packages of ancient volcanic and sedimentary rocks that have been metamorphosed over millions of years.” In other words, they are areas of very old rock that have undergone heat and pressure and have eventually been pushed up toward the surface.
Contrasting models have been proposed to explain the formation of greenstone belts, with the differences between them centering on ideas about plate tectonics. Here’s a brief explanation of the two main camps:
- Some geologists believe that during Archaean times plate tectonics operated differently than they do at present. In their opinion, Archaean plate tectonics centered on mantle plumes, meaning that greenstone belts are oceanic plateaus generated by mantle plumes.
- Others think plate tectonics operated in a similar manner as they do today during the Archaean period. Under this view, greenstone belts would have been formed in volcanic arcs or inter-arc or back-arc basins. They are thus seen as being collages of oceanic crust, island arcs, accretionary prisms and possible plateaus.
However they may be formed, greenstone belts certainly undergo metamorphism, and it is this process that gives them their green color. The low-grade heat that is present when greenstone belts are created results in collections of mostly green minerals, including chlorite, muscovite, albite and epidote.
But where does the gold come from? It is thought that ore-forming fluids may “come from intruded granites or the dehydration of the rocks themselves as they are being metamorphosed.” The fluids redistribute gold along shear zones toward the surface of the Earth. Then, ore deposition occurs when metamorphic fluids mix with water from the surface and drop their gold.
Examples of major greenstone belts
Greenstone belts are found in many parts of the world. As mentioned, Canada’s Abitibi greenstone belt is one well-known example. The Yilgarn craton in Australia is another; both are discussed below.
Abitibi greenstone belt, Canada
The Abitibi greenstone belt spans the Ontario-Quebec border, and is home to many major mines; to date, it has produced well over 160 million ounces of gold. Big companies working in the area include Goldcorp (TSX:G,NYSE:GG), Agnico Eagle Mines (TSX:AEM,NYSE:AEM) and Yamana Gold (TSX:YRI,NYSE:AUY). Exploration is still underway in the area today.
Yilgarn craton, Australia
The Yilgarn craton, Australia’s premier mineral province, receives more than half of Australia’s mineral exploration expenditure, while accounting for two-thirds of all gold production and most nickel production in the country. The craton contains approximately 30 percent of the world’s known gold reserves.
Where could the next big greenstone belt be?
Many greenstone belts are home to significant gold deposits, but could more be found in the future? Geology for Investors notes that experts say there’s more uncharted terrain to be explored, and more greenstone belts to be found. Africa is one place that could have potential.
The continent hosts the Barberton greenstone belt, the Giyani greenstone belt, the Pietersberg greenstone belt, all in South Africa. Then there’s the Gwanda greenstone belt in Zimbabwe, the Lake Victoria greenstone belt in East Africa and finally, the Boromo-Goren greenstone belt over in West Africa.
Where do you think we’ll find the next big greenstone belt? Share your opinion in the comments below.
This is an updated version of an article first published by the Investing News Network in 2011.
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Securities Disclosure: I, Amanda Kay, currently hold no direct investment interest in any company mentioned in this article.