Geologists Discover Innovative Techniques for Copper Ore Extraction

The typical American household contains around 439 pounds, approximately 200 kg, of copper. This copper originates from ore that averages only about 0.6% copper content. Consequently, to supply enough copper for American households, approximately 80,000 pounds, over 36,000 kg, would be required, which translates to more than one truckload. This significant demand keeps geologists focused on discovering new copper deposits.

To locate new copper deposits, geologists need to analyze existing deposits, understand the conditions that generate them, and explore areas with similar geological features. Recently, there has been a renewed focus on studying the conditions responsible for Porphyry copper deposits (PCDs), which provide about 65% of global copper reserves, making them a vital source of copper worldwide.

PCDs form from superheated water associated with large bodies of molten rock. The term is derived from the coarse texture of Porphyry Light mineral. Geologists propose that roughly 70% of known PCDs arise from structural processes termed subduction. Subduction occurs when one tectonic plate is forced under another, at angles ranging from 30° to 60°, leading to the formation of a deep, semi-solid rock wedge known as the mantle. This wedge melts and generates sufficient magma to create igneous rocks related to PCDs, such as granite.

However, researchers believe that the remaining 30% of PCDs might form through different structural processes termed flat slab subduction. In this scenario, one plate slips under another at much shallower angles, typically around 5°, resulting in no mantle wedge formation and therefore no magma production. Consequently, there is a need for a new model to explain the formation of PCDs during flat slab subduction.

Thomas Lamont and his colleagues aimed to develop this model by studying a PCD in central Arizona, where geological activities occurred between 70 and 45 million years ago due to the flat slab subduction of the Farallon Plate beneath North America. To understand the formation of associated PCDs, they analyzed mineral assemblages to determine their ages.

Initially, Lamont’s team dated the PCDs using a red phosphate mineral called Monazite, which contains radioactive uranium and thorium isotopes that decay into lead with known half-lives. By measuring the ratios of these isotopes in monazite, they established that the PCDs likely formed from rocks that melted 730 to 60 million years ago, coinciding with the time of the Farallon Plate’s flat slab subduction.

To gain further insights, the researchers investigated the origins of the sediments. They measured neodymium isotopic values for PCD granites and noted that the mantle has positive neodymium isotopic values above +6, while Proterozoic rocks constituting the North American crust have values between -16 and -18. They found that PCD granites exhibited neodymium isotopic values ranging from -4 to -12.

Lamont and his team then collected and analyzed silicate minerals, specifically zircon, in PCDs, determining the ages of the granites from which they originated. They discovered that zircon came from rocks ranging from 1.2 billion to 2.6 billion years old, matching the age of the Proterozoic crust. Collectively, this evidence suggests that over 70% of PCDs are derived from crustal melting rather than mantle melting.

The team further analyzed granite minerals to ascertain the causes of crustal melting. They observed that the granite minerals differed from older local melts, lacking muscovite and containing reduced amounts of potassium and sillimanite. They interpreted these findings to indicate that melting occurred at water contents of 2.4-3.5 wt%, whereas granite minerals provided only 1.2 wt% water. Thus, they proposed that external water contributed to the melting process.

Based on these observations, the researchers posited that the Farallon Plate underwent fraying and was shallowly submerged under North America, where it experienced considerable pressure. This pressure drove water into the roots of North America, facilitating the melting of the crust and generating sufficient magma for PCD formation.

The researchers concluded that flat slab subduction can enhance the formation of porphyry copper deposits from the crust. This finding reshapes our understanding of copper ore formation and provides a defined new process for geologists to explore and search for large copper deposits in other flat slab subduction regions, potentially addressing the increasing copper demand in society.

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