Blundy, and 6 co-authors, reported in early 2021 that the transition to a low-carbon economy will increase demand for a wide range of metals, notably copper, which is used extensively in power generation and in electric vehicles. Increased demand will require new, sustainable approaches to copper exploration and extraction. Conventional copper mining entails energy-intensive extraction of relatively low-grade ore from large open pits or underground mines and subsequent ore refining. Most copper derives ultimately from hot, hydrous magmatic fluids. Ore formation involves phase separation of these fluids to form copper-rich hypersaline liquids (or ‘brines’) and subsequent precipitation of copper sulfides.
Geophysical surveys of many volcanoes reveal electrically conductive bodies at around 2 km depth, consistent with lenses of brine hosted in porous rock. Building upon emerging concepts in crustal magmatism, we explore the potential of sub-volcanic brines as an in situ source of copper and other metals. Using hydrodynamic simulations, we show that 10 000 years of magma degassing can generate a Cu-rich brine lens containing up to 1.4 Mt Cu in a rock volume of a few km3 at approximately 2 km depth. Direct extraction of metal-rich brines represents a novel development in metal resource extraction that obviates the need for conventional mines, and generates geothermal power as a by-product.
The authors concluded that sub-volcanic regions can trap hypersaline liquids that form on phase separation of ascending supercritical magmatic fluids. There is permissive evidence for the existence of brine lenses from geophysical studies of the roots of active or dormant volcanoes, for example from electrical conductivity surveys. However, these geophysical signals have ambiguities, and further work is required to resolve the contributions to elevated electrical conductivity due to brines, sulfide minerals and silicate melts, and combinations thereof.
Hydrodynamic calculations show that Cu concentrations in dynamic, hypersaline liquid lenses beneath active or dormant volcanoes may be as high as 7000 ppm at temperatures over 400°C. For our reference model, after 10 000 years of degassing the hypersaline liquid lens contains 1.0 to 1.4 Mt of dissolved Cu, depending on the availability of reduced sulfur. These expectations are consistent with limited measurements of high Cu content in fluid inclusions from geothermal drill-core (table 1 in article ), together with analyses of recovered hypersaline fluids at Kakkonda. In order to become potential economic metal resources, hot, metal-rich brines would need to be extracted efficiently without significant reservoir clogging or well-bore scaling. The extent to which such a resource could be exploited depends on the technological challenges and costs of drilling into hot reservoir rocks and recovering hypersaline liquid. If Cu can be extracted in solution form, Cu processing costs may be greatly reduced. Similarly, the co-production of geothermal energy to power fluid extraction and processing would confer additional economic and environmental benefit (e.g. [ see ref 162]).
We have focused only on Cu; the presence of other metals in magmatic fluids, including Li, Zn, Pb, Au and Ag [see ref 4] allows for significant co-recovery. We propose, on the basis of recent developments in our understanding and geophysical imaging of crustal magmatic systems, that mining of sub-volcanic
metal-bearing brines may be a fruitful avenue for addressing the impending growth in demand for certain metal resources that will arise during the transition to a low-carbon economy.
The economic potential of such brines hinges critically on the geophysical challenge of locating them and the technological challenge of extracting them to the surface. Regardless of the economic outcomes, direct drilling into hot, sub-volcanic systems is increasingly feasible technically and will provide invaluable scientific insights into both magmatic and ore-forming processes.
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