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Geologists identify deep earth structures that can signal hidden metal shelters



Geologists identify deep earth structures that can signal hidden metal shelters

A new study shows that the giant mineral deposits are tightly distributed above where the rigid rocks that make up the cores of the ancient continents begin to thin, far below the surface (white areas). The red areas indicate the thinner rocks beyond the border; the bluer ones, the thicker ones. Circles, triangles and squares show large deposits of different metals hosted by sediments. Credit: adapted from Hoggard et al., Nature Geoscience, 2020

If the world is to maintain a sustainable economy and fend off the worst effects of climate change, at least one sector will soon have to accelerate significantly: the extraction of metals necessary to create a vast infrastructure for renewable energy generation, storage, transmission and use. The problem is that the demand for these metals will likely far outweigh both the known deposits and the existing technology used to find more mineral bodies.

Now, in a new study, scientists have discovered previously unrecognized structural lines 1

00 miles or more down the earth that seem to signal the locations of giant deposits of copper, lead, zinc and other viable metals that are located close enough to the surface to extract, but too low to be found using current exploration methods. The discovery could significantly narrow the research areas and reduce the footprint of future mines, say the authors. The study appears in the diary this week Nature Geoscience.

“We cannot move away from these metals, they are in everything and we will not recycle everything that has ever been done,” said lead author Mark Hoggard, postdoctoral researcher at the University of Harvard and Lamont-Doherty of Columbia University. Land. “There is a real need for alternative sources.”

The study found that 85 percent of all known common metal deposits housed in sediment and 100 percent of all “giant” deposits (those containing more than 10 million tons of metal) are above deeply buried lines that surround the planet that mark the edges of the ancient continents. In particular, the deposits are located along the borders where the terrestrial lithosphere, the stiffest external coating on the planet, comprising the crust and the upper mantle, thins out approximately 170 kilometers below the surface.

Until now, all these deposits have been found practically on the surface and their positions seemed to be somewhat random. Most of the discoveries were basically made by geologists who comb the ground and hit the rocks with hammers. In recent decades, geophysical exploration methods have entered that use gravity and other parameters to find buried mineral bodies, 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 is expected to outstrip all base metals hitherto mined in human history over the next 25 years. Copper is used in practically all electronic wiring, from cell phones to generators; lead for photovoltaic cells, high voltage cables, batteries and super capacitors; 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 elements, including cobalt, iridium and molybdenum. A recent study suggests that, in order to develop a sustainable global economy, electric passenger vehicles between 2015 and 2050 must go from 1.2 million to 1 billion; battery capacity from 0.5 gigawatt hour to 12,000; and photovoltaic capacity from 223 gigawatts to over 7,000.

The new study started 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 anything in common. It was based on the fact that in recent years, scientists around the world have used seismic waves to map the highly variable depth of the lithosphere, which extends up to 300 kilometers in the nuclei of the oldest and undisturbed continental masses, and tapers to almost zero under the younger rocks of the ocean floor. As continents have moved, collided and divided by many aeons, their subsurface areas have developed scar-like lithospheric irregularities, many of which have now been mapped.

The authors of the study found that the wealthiest Australian mines are neatly located along the line where the old thick lithosphere extends 170 kilometers as it approaches the coast. They then extended their investigations to around 2,100 sediment-hosted mines around the world and found an identical model. Some of the 170-kilometer boundaries are located near present-day coasts, but many are immersed in the depths of continents, having formed at various points in the distant past when continents had different shapes. Some are up to 2 billion years old.

The scientists map shows these areas that span all continents, including areas of 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, north-western and southern Africa, northern India and central Asia. While some of the identified areas are already home to huge mines, others are complete blanks on the mining map.

The authors believe that metal deposits formed when thick continental rocks extended and subsided to form a depression, like a torn rubber pad. This thinned the lithosphere and allowed seawater to flood. For long periods, these watery low points filled with metal-containing sediments from adjacent rocks at high altitudes. The salt water then circulated down to depths where the chemical and temperature conditions were right for the metals taken from the water in the deep parts of the basin to precipitate out to form giant deposits, anywhere from 100 meters to 10 kilometers below the surface then. The key ingredient was the depth of the lithosphere. Where it is denser, little heat from the warm lower mantle rises to potential mineral formation areas near the surface and where it is thinner, much heat passes. The 170-kilometer border seems to be the area of ​​golden curls to create the right temperature conditions, as long as the right chemistry is also present.

“It really hits the sweet spot,” said Hoggard. “These deposits contain a lot of metal bound in high-quality minerals, so once you find something like this, you only have to dig a hole.” Most of the current base metal mines are sprawling and destructive operations in the open air. But in many cases, deposits as low as one kilometer could probably be mined economically, and these “would almost certainly be drawn through much less destructive trees,” said Hoggard.

The study promises to open up exploration in areas previously unexplored, including parts of Australia, Central Asia and West Africa. Based on a preliminary report from the new study that the authors presented at an academic conference last year, some companies appear to have already claimed ground in Australia and North America. But the mining industry is notoriously reserved, so it is not yet clear how widespread this activity can be.

“This is a truly profound discovery and it is the first time that someone has suggested that mineral deposits formed in sedimentary basins … at depths of only kilometers in the crust were controlled by forces at depths of hundreds of kilometers at the base of the lithosphere”, said a Mining Journal report reviewing last year’s preliminary presentation.

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 Australian National University.

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


Fossilized fish may indicate rich deposits of precious rare earth metals


More information:
Global distribution of metals hosted by sediments controlled by the stability of the skull edge, Nature Geoscience (2020). DOI: 10.1038 / s41561-020-0593-2, www.nature.com/articles/s41561-020-0593-2

Supplied by
Earth Institute at Columbia University




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Geologists identify deep earth structures that can report hidden metal lodges (2020, June 30)
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