EARTH MRI GEOPHYSICAL SURVEY DATA HELP RESOLVE GEOLOGIC MAP BOUNDARY ISSUES IN LESS ACCESSIBLE AREAS: AN EXAMPLE FROM THE WET MOUNTAINS, COLORADO

The USGS Earth Mapping Resources Initiative (Earth MRI) is tasked with improving the Nation’s understanding of critical mineral resources and developing state-of-the-art geologic maps. To help address this goal, a high-resolution airborne magnetic and radiometric survey was flown over the Wet Mountains region in south-central Colorado. The Wet Mountains are composed of a sequence of Proterozoic metavolcanic and metasedimentary rocks intruded by Proterozoic granitic plutons, and by Cambrian intrusive complexes that are spatially associated with fractures and veins enriched in rare earth elements and thorium. The new airborne geophysical data are supporting the Colorado Geological Survey’s geologic map compilation efforts around the Cambrian Democrat Creek complex by extending detailed 1:6,000 scale mapping into less accessible areas mapped in less detail at 1:62,500 map scale, helping to resolve geologic map boundary issues.

The reduced-to-pole magnetic anomaly map shows that both the metavolcanic rocks and the granitic rocks produce linear magnetic highs and lows that parallel measured foliation and follow map-scale folds. Ambiguities in the correlation of these linear anomalies to the mapped rocks can be overcome using the potassium concentration maps, which show elevated values in the granite relative to the metavolcanic rocks. The Democrat Creek complex exhibits a broad magnetic anomaly low, and the radiometric survey shows high concentrations of potassium, thorium, and uranium, both of which extend beyond the surface expression of the complex. Therefore, additional areas of outcrop may have been missed in earlier reconnaissance mapping, and/or the complex extends below the surface. Northwesterly trending faults cross-cut the Precambrian rock fabric at high angle with some exhibiting elevated thorium concentrations. These faults can be extended into areas mapped at smaller scales based on thorium concentrations. However, similarly trending structures exhibit magnetic highs and lows and have no thorium expression. Cambrian dikes mapped in detail have limited expression in the airborne data. Despite these challenges, geophysical observations were successfully used to extend detailed geologic maps into less accessible around the Democrat Creek complex.