Geologists Identify Deep-Earth Structures That Can Point to Hidden Metal Arches

If the world wants to maintain a sustainable economy and fend off the worst effects of climate change, at least one industry will soon have to drastically increase: the metal extraction needed to create a vast infrastructure for the generation, storage, transmission and transmission of renewable energy . use. The problem is that the demand for such metals is likely to far exceed known deposits and the existing technology used to find more deposits.

Now, in a new study, scientists have discovered previously unrecognized structural lines 100 miles or more on Earth that seem to indicate the location of giant deposits of copper, lead, zinc, and other vital metals that are close enough to the surface to extract them. , but too far to be found using current scanning methods. The discovery could greatly reduce search areas and reduce the footprint of future mines, the authors say. The study appears this week in the magazine. Nature Geoscience.

“We cannot escape these metals, they are in everything, and we are not going to recycle everything that was ever done,” said lead author Mark Hoggard, a postdoctoral fellow at Harvard University and Lamont-Doherty at Columbia University. Earth Observatory. “There is a real need for alternative sources.”

The study found that 85 percent of all known base metal deposits housed in sediment, and 100 percent of all “giant” deposits (those containing more than 10 million tons of metal), are on lines deeply buried that surround the planet that marks the edges of the ancient continents. Specifically, the deposits lie along the boundaries where Earth’s lithosphere, the planet’s most rigid outer shell, comprising the crust and the upper mantle, thins about 170 kilometers below the surface.

So far, all of these deposits have been found virtually on the surface, and their locations appear to be somewhat random. Most of the discoveries have been made basically by geologists who comb the ground and hit rocks with hammers. Geophysical exploration methods that use gravity and other parameters to find buried ore bodies have entered in recent decades, but the results have been disappointing. The new study presents geologists with a new high-tech treasure map that tells them where to look.

Due to the demands of modern technology and the growth of populations and economies, the need for base metals in the next 25 years is projected to exceed all base metals mined so far in human history. Copper is used basically in all electronic wiring, from cell phones to generators; lead for photovoltaic cells, high voltage cables, batteries and supercapacitors; and zinc for batteries, as well as fertilizers in regions where it is a limiting factor in soils, including much of China and India. Many base metal mines also produce rarer necessary elements, such as cobalt, iridium, and molybdenum. A recent study suggests that to develop a sustainable global economy, between 2015 and 2050, passenger electric vehicles must increase from 1.2 million to one billion; battery capacity from 0.5 gigawatt hours to 12,000; and photovoltaic capacity from 223 gigawatts to more than 7,000.

The new study began in 2016 in Australia, where much of the world’s lead, zinc and copper is mined. The government funded the work to see if the mines in the northern part of the continent had something in common. It was based on the fact that in recent years, scientists around the world have been using seismic waves to map the highly variable depth of the lithosphere, which stretches up to 300 kilometers in the nuclei of the oldest and most undisturbed landmasses. , and tapers to near zero below the youngest rocks on the ocean floor. As the continents have shifted, collided, and split for many eons, their subsurfaces have developed scar-like lithospheric irregularities, many of which have now been mapped.

The study’s authors discovered that the richest Australian mines were perfectly located along the line where the thick, ancient lithosphere stretches 170 kilometers as it approaches the coast. They then expanded their research to some 2,100 sediment-hosted mines worldwide, and found an identical pattern. Some of the 170-kilometer boundaries lie near today’s shores, but many are located deep within the continents, having formed at various points in the distant past when the continents had different shapes. Some are up to 2 billion years old.

The scientists’ map shows such zones spanning all continents, including 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 South Africa, North India, and Central Asia. While some of the identified areas already host huge mines, others are complete blanks on the mining map.

The authors believe that the metallic deposits formed when thick continental rocks stretched and sank to form a depression, like a separate chewing gum. This diluted the lithosphere and allowed the seawater to flood. Over long periods, these watery low points were filled in with metallic sediments from higher elevation adjacent rocks. The salt water then circulated down to depths where the chemical and temperature conditions were adequate for the metals collected by the water in the deep parts of the basin to precipitate to form giant deposits, anywhere from 100 meters to 10 kilometers beneath the surface. The key ingredient was the depth of the lithosphere. Where it is thicker, little heat from the hot lower mantle rises to potential mineral-forming zones near the surface, and where it is thinner, much heat enters. The 170 kilometer limit seems to be the Goldilocks zone to create the right temperature conditions, as long as the right chemistry is also present.

“It really hits the mark,” said Hoggard. “These deposits contain a lot of metal bound to high-grade minerals, so once you find something like this, you only have to dig a hole.” Most of today’s base metal mines are expanding open pit destructive operations. But in many cases, deposits starting up to a kilometer away could probably be mined economically, and these “would surely be removed via much less disruptive axes,” Hoggard said.

The study promises to open exploration in hitherto under-explored areas, including parts of Australia, Central Asia and West Africa. According to a preliminary report of the new study that the authors presented at an academic conference last year, some companies appear to have claimed ground in Australia and North America. But the mining industry is notoriously secretive, so it is still unclear how widespread such activity could be.

“This is a truly profound finding and it is the first time that anyone suggests that mineral deposits formed in sedimentary basins … at depths of only kilometers in the crust were being controlled by forces at depths of hundreds of kilometers at the base of the lithosphere, “said a report in the Mining Journal reviewing the preliminary submission last year.

The other authors of the study are Karol Czarnota of Geoscience Australia, who led the initial Australian mapping project; Fred Richards of Harvard University and Imperial College London; David Huston of Geoscience Australia; and A. Lynton Jaques and Sia Ghelichkhan of the National University of Australia.

Hoggard has put the study in a global context on his website: https://mjhoggard.com/2020/06/29/treasure-maps