Preliminary geophysical interpretations of regional subsurface geology near the Questa Mine Tailing Facility and Guadalupe Mountain, Taos County, New Mexico

This report presents geophysical interpretations of regional subsurface geology in the vicinity of the Tailing Facility of the Questa Mine near Guadalupe Mountain, Taos County, New Mexico, in cooperation with the New Mexico Environment Department. The interpretations were developed from aeromagnetic data, regional gravity data, data from four ground magnetic traverses, geologic mapping, a digital elevation model, and information from a few shallow wells. The resolution of the geophysical data is only appropriate for a broad assessment of the regional setting. Aeromagnetic data provided the most comprehensive information for interpretation. Qualitative and semiquantitative interpretations indicate the nature and extent of volcanic rocks, their relative depths, and inferred contacts between them, as well as conjectured locations of faults. In particular, the aeromagnetic data indicate places where volcanic rocks extend at shallow depths under sedimentary cover. Trachydacites of Guadalupe Mountain are magnetic, but their associated aeromagnetic anomalies are opposite in sign over the northern versus the southern parts of the mountain. The difference indicates that lavas erupted during different magnetic-polarity events in the north (reverse polarity) versus the south (normal polarity) and therefore have different ages. We postulate a buried volcano with reverse-polarity magnetization lies under the northeast side of Guadalupe Mountain, which likely predated the exposed trachydacites. Faults interpreted for the study area generally align with known fault zones. We interpret a northern extension to one of these faults that crosses northwesterly underneath the Tailing Facility. Gravity data indicate that Guadalupe Mountain straddles the western margin of a subbasin of the Rio Grande rift and that significant (>400 meters) thicknesses of both volcanic and sedimentary rocks underlie the mountain.