INTRUSIONS REVEALED BY A NEW EARTH MRI AIRBORNE GEOPHYSICAL SURVEY IN SOUTHWESTERN 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 2023, an Earth MRI airborne magnetic and radiometric survey was flown in southwestern Colorado as part of an ongoing project to investigate the Colorado Mineral Belt. The survey was flown along north-south lines spaced 200 m apart with a nominal terrain clearance of 100 m.

Previous geologic mapping identified many Late Cretaceous to Oligocene intrusions that penetrate a thick Phanerozoic sedimentary section in the far southwestern part of the survey area (at the western edge of the San Juan Mountains south of 37°45’ N). Erosion of the sedimentary cover also discloses a Proterozoic gabbro in the Electra Lake area. Critical minerals of interest to Earth MRI (barite, bismuth, manganese, palladium, platinum, tellurium, and zinc) occur in association with precious- and base-metal deposits that have been mined from veins associated with the intrusions since the 1900s.

Aeromagnetic data are effective for imaging the intrusions at depth, owing to the moderately to highly magnetic intrusive rocks that contrast with the effectively nonmagnetic sedimentary section. The improved resolution of the Earth MRI aeromagnetic data compared to previously available data reveal numerous dikes that are completely buried or extend farther than they are mapped at the surface. The dikes are oriented in multiple directions and some of them cross other dikes. Many extend >10 km in length. Broad aeromagnetic anomalies encompass disconnected exposures of most of the larger intrusions, defining their lateral extents at depth. Evidence of highly magnetic rocks in isolated areas near the surface attest to the complexity of the magmatic and/or hydrothermal systems. Strong magnetic anomalies correspond to the exposed Proterozoic gabbro at Electra Lake and transition to broad anomalies that extend outward over mapped sedimentary rocks for 8 km both to the east and west. These relations suggest the gabbro is much more extensive below the sedimentary cover.