epithermal

Epithermal is an adjective used in geology and nuclear physics to describe regimes that are intermediate between two well-defined states. The term is commonly applied to ore deposits and to neutron energy ranges.

In geology, epithermal ore deposits are hydrothermal veins formed at relatively shallow depths in volcanic or volcanic-hosted environments. They develop as hot fluids rise toward the surface and precipitate metals when pressure and chemistry change. Epithermal deposits are often rich in precious metals such as gold and silver and are subdivided into low-sulfidation and high-sulfidation styles, distinguished by fluid composition, sulfide content, and alteration minerals. Mineral assemblages commonly include quartz, adularia, calcite, and clays, with textures such as veins and stockworks. Deposition temperatures typically fall in a broad range around 50–200°C, and these systems are frequently linked to recent volcanic activity.

In neutron science and medical physics, epithermal neutrons are those with energies above thermal but below fast neutrons. A commonly cited range is roughly 0.5 eV to tens of keV. Epithermal neutrons penetrate tissue more effectively than thermal neutrons and then slow to thermal energies within the body, which can enhance certain nuclear reactions. This property is exploited in therapies such as boron neutron capture therapy (BNCT), where boron-10 captures an epithermal neutron and releases high-LET particles that damage nearby cells.

Overall, epithermal denotes an intermediate regime, whether describing the depth and conditions of hydrothermal ore formation or the energy range of neutrons used in scientific and medical applications.