The average American home contains 439 lbs, or nearly 200 kgs, of copper. This copper comes from mined ore, which only contains about 0.6% copper on average. Therefore, it takes nearly 80,000 lbs, more than 36,000 kgs, or an 18-wheeler’s weight of rock to provide enough copper for an American family. This high demand makes finding new copper deposits a top priority for geologists.
To find new copper deposits, geologists study known deposits to determine the conditions that formed them, then look in places with similar conditions. Recently, geologists reexamined the conditions that form porphyry copper deposits, or PCDs. PCDs contain about 65% of global copper reserves, making them the world’s most important source of copper.
PCDs are formed by superheated water from massive bodies of molten rock. They’re named for their coarse-grained, or porphyritic, mineral texture. Geologists think approximately 70% of known PCDs formed through a tectonic process called subduction. In typical subduction, one tectonic plate is pushed beneath another and into the Earth at a 30॰ to 60॰ angle, forming a wedge of deeper, semi-solid rock called mantle between them. This mantle wedge then melts, producing enough magma to create the PCD and an associated body of igneous rocks, called granite.
However, researchers hypothesize that the remaining 30% of PCDs could have formed during a different tectonic process called flat slab subduction. Here, one plate subducts below another, but at a much shallower angle, typically 5°. This causes it to scrape underneath the plate above it instead of forming a mantle wedge. Without a mantle wedge, no melting occurs, and no magma is produced. Thus, geologists need a new model for how PCDs formed during flat slab subduction.
Thomas Lamont and colleagues set out to define this model using PCDs in central Arizona. The geology of this area was produced during flat slab subduction of the Farallon plate below North America from 70 to 45 million years ago. To explore how associated PCDs formed, the researchers analyzed their mineral assemblages and determined their specific ages.
First, Lamont and colleagues established when the PCDs formed. They used a red phosphate mineral called monazite that contains radioactive uranium and thorium isotopes, which decay into lead at a known half-life. By measuring the ratios of these isotopes in the monazites, Lamont established that the PCDs formed from rock melted between 73 and 60 million years ago. This timing coincided with the Farallon plate’s flat slab subduction, meaning it likely played a role in their formation.
To better understand this role, the researchers next determined where the deposits originated. They compiled the neodymium isotope values of PCD granites, which indicated where the molten rock originated. They explained that the mantle has positive neodymium isotope values of greater than +6, while the Proterozoic rocks that make up the North American crust have negative neodymium isotope values of -16 to -18. They found the PCD granites all had negative neodymium isotope values of -4 to -12, meaning they came from the crust.
Lamont and his group then collected and analyzed silicate minerals called zircons in the PCDs, which preserved the age of the granites they came from. They found that the zircons originated from 1,200 to 2,600 million-year-old rocks, matching the age of the Proterozoic crust. Taken together, they interpreted this evidence to indicate that over 70% to 90% of the PCDs came from the crust melting instead of the mantle melting.
The researchers then analyzed minerals in the granite to determine what caused the crustal melting. They found that the granites had different minerals than older local melts – they were devoid of muscovite and had less potassium-feldspar and sillimanite. They interpreted these minerals and their ratios to mean that melting occurred with 2.4 to 3.5 wt% water, but the granite’s minerals only provided 1.2 wt% water. Therefore, they suggested that water must have been added to the rock, which caused melting.
Based on these observations, the researchers suggested that the Farallon plate subducted shallowly, scraping under North America. There, it experienced extreme pressure, which forced water out of it and into the base of North America. The water caused the crust to melt and created enough magma to form the PCDs.
The researchers concluded that flat slab subduction can drive the formation of porphyry copper deposits from the crust, which changes our understanding of copper ore formation. With this new process defined, geologists can now look for major copper deposits in other areas of flat slab subduction, which could help meet society’s growing copper demands.
