In a paper published in the journal Nature Geoscience, scientists at Harvard University, Columbia University, Geoscience Australia and the Australian National University say this discovery could greatly narrow down search areas, and reduce the footprint of future mines.
In detail, the study found that 85% of all known base metal deposits hosted in sediments —and 100%t of all deposits that hold more than 10 million tonnes of metal— lie above deeply buried lines girdling the planet that mark the edges of ancient continents.
The deposits lie along boundaries where the Earth’s lithosphere thins out to about 170 kilometres below the surface. Up till now, all such deposits have been found at the surface, and their locations have seemed to be somewhat random.
To find them, geologists normally hammer rocks or use geophysical exploration methods that entail harnessing gravity and other parameters to find buried ore bodies. The new study proposes a new, high-tech treasure map that provides more accurate information so that prospectors know where to look.
To build such a map, the researchers build on existing charts created using seismic waves that reveal the highly variable depth of the lithosphere, which ranges down to 300 kilometres in the nuclei of the most ancient, undisturbed continental masses, and tapers to near zero under the younger rocks of the ocean floors.
“As continents have shifted, collided and rifted over many eons, their subsurfaces have developed scar-like lithospheric irregularities, many of which have now been mapped,” the researchers said in a media release.
Using this information, the authors of the study found that the richest Australian mines lay neatly along the line where thick, old lithosphere grades out to 170 kilometres as it approaches the coast. They then expanded their investigation to some 2,100 sediment-hosted mines across the world, and found an identical pattern. Some of the 170-kilometre boundaries lie near current coastlines but many are nestled deep within the continents, having formed at various points in the distant past when the continents had different shapes.
Their new map shows such zones looping through areas in western Canada, the coasts of Australia, Greenland and Antarctica; the western, southeastern and Great Lakes regions of the United States; and much of the Amazon, northwest and southern Africa, northern India and central Asia. While some of the identified areas already host enormous mines, others are complete blanks on the mining map.
“These deposits contain lots of metal bound up in high-grade ores, so once you find something like this, you only have to dig one hole,” said Mark Hoggard, lead author of the paper and a postdoctoral researcher at Harvard University and Columbia University’s Lamont-Doherty Earth Observatory.
“Most current base-metal mines are sprawling, destructive open-pit operations. But in many cases, deposits starting as far down as a kilometre could probably be mined economically, and these would almost certainly be taken out via much less disruptive shafts.”
In Hoggard’s view, this study promises to open exploration in poorly explored areas, including parts of Australia, central Asia and western Africa.