GEOPHYSICAL INVESTIGATIONS OF THE MOUNT BAKER GEOTHERMAL PLAY

Phase II of a geothermal play fairway exploration was conducted at three plays (Mount Baker, Mount St. Helens seismic zone, and Wind River valley) along the Washington Cascade Range. The goal of phase II exploration is to collect geologic and geophysical data to help constrain spatial changes in subsurface lithology and structures that may be associated with the geothermal system. The Mount Baker play is an ideal location for further investigation as a potential geothermal resource due to the presence of thermal features, young volcanic centers, and elevated infrastructure favorability. We evaluate the geothermal potential of the southeast flank of Mount Baker by interpreting newly collected high-resolution gravity, magnetotelluric (MT), and ground-magnetic data. We collected 495 gravity stations, over 93 line-km of ground magnetic data, and 28 MT stations within the study area.

3D resistivity modeling revealed a 2-3 km³ conductive zone which nears the surface ~200 m east of the Baker Hot Springs and has a resistivity value similar to that of the hot spring water. The conductive zone extends to over 3 km depth and is interpreted as a volume of upwelling hot fluid that ascends along a steeply north-dipping fault. Ground magnetic data revealed a NNE-trending 1.7-km-long highly magnetic body that runs through the hot springs and is in line with previously mapped lineaments identified in LiDAR. The body was inversely modeled in 3D producing a 2 km³ roughly tabular body which roots down to the NW near the conductive MT anomaly. The results of potential field forward modeling suggest that this feature is consistent with being either a low conductivity mafic intrusion, which may be fault-controlled or a steeply SE-dipping fault, juxtaposing basement rocks with sharply contrasting magnetic properties to the west and east of the modeled fault. The fault model is preferred as it provides the simplest solution which agrees with available observations. This feature is intercepted by a steep NE-trending magnetic gradient ~0.5 km north of the hot springs. Interaction of the structures may play an important role in generating permeability near the springs, providing a pathway for thermal fluids to the surface. These data resolve key structural features and provide information valuable for guiding future exploration of the Mount Baker play.