Heat and mass transfer by hydrothermal fluid flow in a caldera setting

Unravelling heat and mass transfer in caldera volcanoes is critical to understanding the formation of geothermal and mineral resources. While geophysical monitoring and field work provide snapshots of the hydrothermal system, numerical simulations can quantify their dynamic behavior over time. We present three-dimensional fluid flow simulations of a cooling oblate intrusion in a caldera setting to explore the effect of a high-permeability ring fault on flow dynamics and heat and mass transfer. Hot hydrothermal systems develop only if the host rock has sufficient permeability to allow for fluid advection, which efficiently extracts heat from the intrusion. In essentially impermeable host rock, fault-bound hydrothermal circulation reaches only moderate temperatures, as heat extraction at depth is limited by conductive heat transfer. In permeable host rocks, the position and thermal structure of the hydrothermal system are strongly transient, shifting from mostly fault-hosted to caldera-infill-hosted as the system evolves. The primary control on this behavior is inward intrusion cooling, with hydrothermal upflow zones initiating along the intrusion margin and migrating inward as intrusion cooling progresses. Upflow within the ring fault is only significant while the fault coincides with such a natural upflow zone, during which it can strongly focus heat and mass transfer. Partitioning of heat and mass transfer between the fault and the caldera infill therefore changes with time as the heat source shrinks inward. Overall, the hydrothermal system evolves similarly with or without a ring fault, except during the early stages when flow through the ring fault can dominate. • Three-dimensional modelling of magmatic heat-driven fluid flow in a caldera volcano. • Ring faults can transiently localize heat and mass transfer when aligned with natural upflow zones. • Upflow zones shift from the ring fault into the caldera infill as intrusion cooling progresses and the heat source shrinks. • Sufficient bulk permeability of the surrounding rocks rather than the fault is crucial to allow heat extraction from the cooling intrusion.