In September, a group of advocates concluded a months-long “prayer journey” from the Lummi Nation north of Seattle to Washington, D.C., to deliver an appeal to the United States Supreme Court. The appeal asks the Supreme Court to hear a case determining whether the government is violating the constitution by swapping government-owned land with Resolution Copper, a joint venture of two mining companies, Rio Tinto and BHP, for the development of a copper mine near Superior, Ariz.
The advocates, from the nonprofit Apache Stronghold, are the plaintiffs in the case. In Apache Stronghold v. United States of America, they argue that the development of a copper mine would violate the First Amendment rights of Indigenous community members who consider Oak Flat, a 4,600-acre area above the copper deposit, an important religious site, said Naelyn Pike, a spokesperson for Apache Stronghold.
The area is one of many in southeastern Arizona that, by a twist of geologic fate, sit above significant copper deposits, making them magnets for mining operations.
The Copper Triangle Emerges
Arizona is home to some of the world’s largest copper deposits, giving it the nickname “the Copper State.”
Some of these deposits exist in the so-called Copper Triangle, southeast of Phoenix, which formed after magma intruded into the upper 8–10 kilometers (5–6 miles) of crust.
These intrusions may have been fed from a larger, deeper body of magma created when the Farallon plate was subducting beneath the North American plate during the Laramide orogeny, though their source is still debated by scientists.
The magma intrusions, like most ordinary igneous bodies of their size, carried dissolved metals, including copper. As the magma cooled in the shallow crust and minerals started to crystallize, highly pressurized, hot, metal-rich fluids were abruptly pushed into the surrounding rock along fractures. The fluid depressurized, cooled, and reacted with the rock around it to precipitate copper in the form of copper-iron sulfides such as chalcopyrite.
Deposits such as these are known as porphyry deposits.
“You can think of it as a volcanic eruption in the subsurface,” said Mark Barton, a geologist at the University of Arizona.
Analysis of sediment cores from the region indicates that three of these hydrothermal eruptions probably took place during the Late Cretaceous, about 65 million years ago.
The Copper Triangle “is really fertile for porphyry deposits.”
The Copper Triangle “is really fertile for porphyry deposits,” said Adam Simon, an economic geologist at the University of Michigan.
Geologic activity continued after the porphyry copper deposits formed. Over time, extensional faulting caused by a stretching of the crust chopped up the deposits and caused blocks of rock to tilt like toppling dominoes, which moved the deposits closer to the surface. About 18 million years ago, an eruption from the nearby Superstition caldera covered the area with volcanic tuff (ash), forming what is now Oak Flat.
The top of the copper ore body is currently 1.5 kilometers (5,000 feet) below the surface at its shallowest.
Though ancient magmatism formed deposits throughout the West, southeastern Arizona is unusually convenient to mine, Barton explained. The underground copper has been lifted close enough to the surface to feasibly access, but is still buried deep enough to be preserved. That’s thanks to the climate in southwestern North America, which has been relatively dry for much of the past 200 million years, preventing erosion from exposing the deposits.
High-Grade Potential
According to Resolution Copper, the grade of the copper deposit at Oak Flat is quite high, at 1.5% copper. That’s exceptional, said Barton and Simon, as most copper deposits in the southwestern United States are typically about 0.5% copper.
“It’s very large and it’s exceptionally rich…certainly in the top 20 of these kinds of deposits around the world in terms of size and contained metal.”
Because it costs the same amount of money to mine a high-grade deposit as a low-grade deposit, the Resolution site is particularly attractive for industrial activity, Barton said. “It’s very large and it’s exceptionally rich…certainly in the top 20 of these kinds of deposits around the world in terms of size and contained metal.”
The high amount of copper at the site may also reduce the need to mine elsewhere, he said. Resolution Copper claims it will be able to produce as much as 18 billion kilograms (40 billion pounds) of copper over 40 years—enough for more than 200 million electric vehicle engines, according to the International Copper Association.
To access the ore, Resolution Copper proposed to use a method called panel caving or block caving, in which large sections of underground rock are cut and removed. The process eventually causes the surface to subside, permanently altering the land above.
“There will be a large pit that is generated at the surface by the mining as a result of pulling this large volume of material out from underneath it,” Barton said.
Resolution Copper has said it will maintain public access to Oak Flat but did not respond to requests for further comments in time for publication.
—Grace van Deelen (@GVD__), Staff Writer
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Citation: van Deelen, G. (2024), Here’s why Resolution Copper wants to mine Oak Flat, Eos, 105, https://doi.org/10.1029/2024EO240459. Published on 16 October 2024.
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