Assessing potential ‘copper in place’ in subvolcanic brines

Global copper demand is projected to increase from 22.8 Mt in 2024 to 35 Mt by 2040, driven largely by the transition to green energy technologies. Existing and announced Cu mining projects are forecast to meet only 70% of this demand by 2035, creating a significant supply deficit. Mining of subvolcanic magmatic brines - hypersaline and potentially supercritical fluids enriched in metals – has been proposed as an alternative source (Blundy et al., 2021). Here, we assess the potential mass of Copper Initially in Place (CIIP) in such reservoirs.

Based on published resistivity models from 46 active magmatic-hydrothermal systems, we estimate the typical volume of brine reservoirs to range from 10 to 200 km³ and the average top reservoir depth to be 1.7 km, well within reach of modern drilling technology. Typical reservoir porosity in the shallow sub-critical zone is 8±6% and decreases to 3±3% in the deeper supercritical zone. Copper concentration in the brines is the most uncertain property.  Data from fluid inclusions and Cu solubility modelling suggest that most brine reservoirs will host modest Cu concentration (ca. 10’s to 100’s ppm), but values could exceed 10,000 ppm in the most Cu enriched systems.

We combine these estimates of reservoir volume, porosity and copper concentration using a probabilistic Monte Carlo framework to provide estimates of CIIP. Our analysis indicates a lognormal CIIP distribution with a median (P50) of 8.6 Mt and a P90 of 55 Mt, suggesting that individual magmatic brine resources may be comparable in size to conventional copper porphyry deposits. Moreover, a single high-flow-rate well tapping into a supercritical reservoir could produce approximately 2.4 kt of copper per year. A large-scale operation comprising multiple wells could yield 0.24 Mt/year, equivalent to roughly 1% of current global demand.

A Cu brine mine could extract geothermal energy from the produced fluids. We envisage a self-powered Cu brine mine, with net positive energy per kg of Cu and a minimal environmental footprint. While significant challenges remain regarding exploration for copper-rich brine reservoirs and production of very hot and possibly supercritical brines, brine mining offers a potentially significant source of Cu that could be produced with much lower energy demand and negative environmental impact than conventional mining.

Blundy, J., et al. "The economic potential of metalliferous sub-volcanic brines." Royal Society Open Science 8.6 (2021): 202192.