Jim Mungall, a U of T geology professor, thinks he can explain the presence of gold and copper deposits around some volcanoes and their absence from others. His findings give miners a new way of guessing where they might strike it rich.
The majority of gold and copper deposits are found over subduction zones. These are places where slabs of cold, oxidized rock from the ocean floor descend beneath chains of volcanoes that mark the edge of another slab.
Deep under the Earth’s crust, grains of sulphide (a compound of sulphur) are dispersed throughout the rock. These particles hold on to gold and keep it from rising to the crust where it is accessible to miners. The minerals are trapped, and when the magma underneath a volcano rises to the surface, the gold and copper are left behind.
“The key is to remove sulphide from the source” so the copper and gold can escape, Mungall explained. Ferric iron helps to oxidize the sulfide to a different form, called sulphate, which allows the metals to dissolve in molten rock and be carried up to the surface.
Mungall’s theory hinges on the presence of ferric iron in areas deep under the Earth. This in turn depends on whether the descending slab of ocean floor melts at the subduction zone, since a descending slab is the only medium that can transport ferric iron to areas where gold exists deep underground. In most cases the slab does not melt since it doesn’t get hot quickly enough.
There are three ways in which slab melting may occur. The slab may become stuck and jam as it descends, giving it more time to heat up, or it may be unusually hot to begin with. Or the slab may be driven horizontally under a volcano. But each of these phenomena occurs very rarely.
The important question is: How often do these slabs actually melt? As Mungall explained, “Everyone agrees it does occasionally happen. [There is] disagreement about how often.”
Mungall describes his work as “a means to discriminate potentially fertile and barren volcanic or intrusive complexes.” In other words, his theory helps to tell where the gold is.
By examining samples of rocks at a specific volcanic complex for the chemical elements they contain, a geologist could tell if slab melting had taken place and determine if the site is likely to contain deposits of gold.
The findings were published in the October issue of Geology.
Photograph by Ryan Clements
