Magnetotelluric imaging of the magmatic and geothermal systems beneath Mount Meager, southwestern Canada

Abstract

Magnetotelluric data were collected on and around the Mount Meager Volcanic Complex, an active volcanic complex with eruptions ~2,400 and ~24,300 years ago. 3-D inversion was used to create an electrical resistivity model to a depth > 20 km. The model is characterized by high resistivity (> 100 Ωm) in the upper 6-7 km, implying relatively dry, unaltered rock. Within this resistive layer, localized conductors are observed in the upper 2 km beneath Pylon Peak and Fish Creek, corresponding to low-permeability clay-rich layers, acting as caprocks to geothermal fluids below. Beneath the resistive upper crust, there is a large conductor at ~5-15 km below sea level with an average resistivity of ~3 Ωm. Laboratory experiments of melt resistivity and petrological data from erupted volcanic rocks were used to interpret the model. The magma body (deep conductor) is inferred to have a minimum volume of ~2×10^12 m^3 comprising ~18-32% dacitic-to-trachydacitic melt with ~6-8 wt% H2O at a temperature of ~800-900ºC. This is below the melt fraction of an eruptible magma body. Resolution tests suggest it might be regional in extent, not localized beneath Mount Meager. There are fluid pathways from the northern part of the magma body, up toward Mount Meager and nearby fumaroles. This model is a significant advancement from its predecessors, created 20-40 years ago, providing the first deep 3-D image of this volcanic system. Along with other geophysical and geological models of the Garibaldi Geothermal Energy Project, it will reduce the exploration risk associated with geothermal energy development.