ESTIMATING THE GEOMETRY AND EXTENT OF ALTERATION IN HYDROTHERMAL AREAS OF YELLOWSTONE NATIONAL PARK BASED ON HIGH-RESOLUTION MAGNETIC SURVEYS

Yellowstone National Park (YNP) hosts extensive hydrothermal activity. Although hydrothermal alteration in YNP has been previously studied, the volume, geometry, and type of rock alteration at depth remain poorly constrained. Magnetic surveys can help to investigate buried hydrothermal alteration as demonstrated by the high-resolution aeromagnetic survey of YNP (flight line spacing < 500 m and flight elevation <350 m above ground; Finn and Morgan, J. Volcanol. Geotherm. Res., 115, 207–231, 2002). This survey displayed magnetic lows extending over and beyond areas of hydrothermal activity suggesting large volumes of buried demagnetized rocks due to hydrothermal alteration of the volcanic units. In order to better understand the relationship between magnetic lows and hydrothermal alteration, we performed ground-based magnetic surveys using a cesium-vapor magnetometer within three thermal areas in YNP (Norris Geyser Basin, Lone Star Geyser Basin, and Smoke Jumper Hot-springs). These surveys include several 4-5 km long transects and a detailed survey over an area of about 800 m x 500 m around Lone Star Geyser. In addition, we performed laboratory measurements of magnetic susceptibility, magnetic remanence, and density on rock samples collected in the field and from drill cores collected in 1967-1968 to characterize physical properties of fresh and altered geologic units. The ground surveys show that short-wavelength anomalies are damped over hydrothermal alteration areas and that a pronounced negative anomaly is centered on the Lone Star geyser mound. The later indicates that significant demagnetization of the volcanic substratum is associated with hydrothermal alteration and in particular areas with large-scale hydrothermal flow. Ground magnetic profiles are used to estimate the depth to magnetic sources based on the maximum value of its second derivative. Aeromagnetic data and the detailed ground surveys are then used to invert the depth to the top of the magnetic substratum. Together, these methods suggest that significant demagnetization occur over a thickness of a few hundred meters in hydrothermal areas at YNP.