Volcanoes are intertwined with human life since prehistoric times. Whereas on one hand they may be source of destruction (Figure 1) on the other they provide elements that are essential to humans, like potassium and magnesium which are fertilizers of soils for agricultural crops. Perhaps less known, volcanoes are also intimately associated with the major sources of copper on Earth. Copper has been one of the most important metals for human civilization and, nowadays, is essential for the transition to a green economy. It is estimated that its demand will overcome known natural resources within a few decades, which is fostering efforts to find new resources.
The largest natural copper resources are the so-called porphyry copper deposits (Figure 2). Most of these deposits form 1-6 km under volcanoes associated with subduction zones, like those of the Andean Cordillera in South America. The magma reservoir under volcanoes may feed explosive eruptions through the catastrophic liberation of fluids inside the magma ascending towards the surface, but may also release fluids in a quieter way. Such fluids consist principally of water with trace amounts of copper and other metals. Copper is precipitated as copper-rich sulfide minerals, like chalcopyrite (CuFeS2), due to cooling of the fluids released by the magma reservoir when they ascend towards the surface.
Despite our understanding of porphyry copper deposits has significantly progressed during the last decades, little attention has been given to the quantification of the processes leading to the formation of porphyry copper deposits with a range in copper endowments of more than three orders of magnitude (<1 Mt to >100 Mt Cu). No need to say that this would be beneficial for a cost-efficient exploration of porphyry copper deposits.
Since a few years, together with my colleague Luca Caricchi also at the University of Geneva, I am trying to quantify intensive and extensive variables that lead to the broad range of endowments of porphyry copper deposits. The approach we use is mass balance combined with petrological and geochemical modelling. From a mass balance perspective, the main parameters that control the size of porphyry copper deposits are the amount of aqueous fluid, its copper concentration, and the efficiency of precipitation of copper from such a fluid.
Precipitation efficiency is considered to be broadly similar in porphyry copper deposits. Also copper contents of magmatic fluids vary within a relatively narrow range, based on measurements of natural fluids and laboratory experiments. This has brought us to quantify the link between potential copper endowment of a porphyry copper deposit and amount of fluid that has precipitated copper in the deposit. The amount of such fluid, in turn, depends on the amount of magma that ultimately releases the fluid. Using this rationale, we calculated that supergiant porphyry copper deposits, containing at least 10 Mt Cu and up to >100 Mt Cu, require an amount of magma that is at least ~500 km3 and up to >2500 km3. Previous studies suggest that these magma volumes, which are similar to those erupted catastrophically during large eruptions, can only accumulate in large regions of the lower continental crust at depths of a few tens of km. However, at these depths, magmas cannot exsolve an aqueous fluid, which is a necessary step to form porphyry copper deposits, as mentioned above. This is because, like carbon dioxide in sparkling water, also the solubility of water in magma strongly increases with pressure, i.e., with depth in the crust. In order to release its fluid cargo, the magma accumulated in the deep crust needs to ascend to shallower levels. Here, water solubility decreases and the fluid is liberated together with copper, because the latter preferentially goes into the fluid phase rather than remaining in the silicate melt. The transfer of magma from the deep to the shallower crust, where porphyry copper deposits form, occurs through a transcrustal magmatic system (Figure 3) within timescales that are constrained by the duration of the process of fluid exsolution and metal precipitation. Such timescales can be precisely measured using natural radioactive decay of U and Re in two minerals associated with the formation of porphyry copper deposits, zircon (ZrSiO4) and molybdenite (MoS2) respectively. We have calculated the rate of magma transfer from the deep to the shallower crust, which can be expressed as km3 of magma transferred in a year, and realized that these transfer rates were very high and similar to those usually leading to large eruptions. This was unexpected because, as it can be inferred intuitively, an eruption, especially a large one, will destroy the porphyry deposit situated between the shallow magma reservoir and the volcano (Figure 3). On the other hand, this conclusion suggests that there are peculiar conditions for which high-rate magma injection beneath a volcano may lead to the growth of a large magma reservoir without a catastrophic eruption, allowing a "quiet" release of copper-bearing fluids. The main conclusion of our work is that magmatic systems leading to the formation of either porphyry copper deposits or catastrophic large eruptions, are, ultimately, very, albeit not entirely, similar (compare panels a and b in Figure 3). This finding opens new avenues in the development of geological, mineralogical, and geochemical tools for the successful exploration of the largest porphyry copper deposits on Earth.