Аннотация:Many active or dormant volcanoes show regions of high electrical conductivity at depths of a few kilometres beneath the edifice. We explore the possibility that these regions represent lenses of high-salinity brine separated from
a single-phase magmatic fluid containing H2O and NaCl. Since the chloride-bearing fluids have an exceptional capacity to transport metals and are highly conductive, the regions can be an indicator of a hydrothermal ore deposits
located or forming beneath volcanoes. To investigate this possibility we have performed hydrodynamic simulations
of magma degassing into permeable rock. In our models the magma source is located at 7 km depth and the fluid
salinity approximates that expected for fluids released from typical arc magmas. Our model differs from previous
models of a similar process because it is (a) axisymmetric and (b) includes a static high-permeability pathway that
links the magma source to the surface. This pathway simulates the presence of a volcanic conduit and/or plexus of
feeder dykes that are typical of most volcanic systems. The presence of the conduit leads to a number of important
hydrodynamic consequences, not observed in previous models. Importantly, we show that an annular brine lens
capped by crystallised halite is likely to form above an actively degassing sub-volcanic magma body and can persist
for more than 250 kyr after degassing ceases. Parametric analysis shows that brine lenses are more prevalent when
the fluid is released at temperatures above the wet granite solidus, when magmatic fluid salinity is high, and when
the high-permeability pathway is narrow. The calculated depth, form and electrical conductivity of our modelled
system shares many features with published magnetotelluric images of volcano subsurfaces. The formation and
persistence of sub-volcanic brine lenses has implications for geothermal systems and hydrothermal ore formation,
although these features are not explored in the presented model.