EARTH MRI AIRBORNE GEOPHYSICAL SURVEYS ENABLE BETTER GEOLOGIC INTERPRETATION: EXAMPLES FROM OLD AND NEW AIRBORNE MAGNETIC AND RADIOMETRIC SURVEYS OVER THE WET MOUNTAINS REGION, SOUTH-CENTRAL COLORADO

Acquisition of high-resolution airborne magnetic and radiometric surveys is a major component of the USGS Earth Mapping Resources Initiative (Earth MRI). The purpose is to provide modern, high quality geophysical data that can be used to better understand the geologic framework in areas that have potential for hosting critical minerals. In 2021, an Earth MRI airborne survey was flown over the Wet Mountains and vicinity, including parts of Custer and Fremont counties in south-central Colorado. The Proterozoic terrane of this region is intruded by several Cambrian alkaline complexes that are spatially associated with veins and fracture zones enriched in rare earth elements (REE) and thorium (Th).

The Earth MRI Wet Mountains survey, covering 2,200 square km, was flown along E-W flight lines spaced 150 m with variable terrain clearances that averaged 122 m over the whole area. Previous public data for the Wet Mountains region are available from a 1968 USGS aeromagnetic survey that was digitized from paper maps and a combined magnetic and radiometric survey flown as part of the National Uranium Resource Evaluation (NURE) program in the 1970s. Both previous surveys employed flight lines spaced more than 5 times wider than those of the Earth MRI survey. The wide spacing fails to properly sample geophysical signals over large swaths of ground in between flight lines, which degrades resolution needed for geologic interpretation.

Notable examples of improved resolution of the Earth MRI data that enables better geologic interpretation are (1) numerous expressions of the Th-enriched veins in the equivalent Th data that follow and augment the locations previously mapped on the ground; (2) magnetic and radiometric expressions of the alkaline complexes that reflect their differing rock compositions; (3) details that distinguish the sources of previously similar-looking, large magnetic highs in other areas as deep versus shallow bodies, one of which may be a previously unrecognized Cambrian mafic intrusion; (4) detailed expressions of the complicated Proterozoic tectonic fabric and cross-cutting northwest-striking faults, which are generally oblique to the Th-enriched veins and fractures; and (5) largely concealed faults attributed to activity on the Miocene Rio Grande rift to the west that can be delineated on the magnetic map.