WHAT DO GEOPHYSICAL INVESTIGATIONS OF THE BEAR LODGE, WYOMING, RARE EARTH ELEMENT DEPOSIT REVEAL?

An igneous complex that forms the core of the Bear Lodge Mountains in northeast Wyoming hosts numerous mineral deposits of economic importance. Eocene-age alkaline intrusions in Paleozoic and Mesozoic sedimentary rocks formed a dome about 13 km long by 10 km wide. The carbonatite-hosted rare earth element deposit was identified in the early 1950s following a period of fluorite mining and uranium prospecting. Occurring as veins and disseminated deposits, total rare earth oxides at this site are estimated to be approximately 380,000 metric tons.

The U.S. Geological Survey has been analyzing and interpreting geophysical datasets over rare earth element deposits in order to characterize the geologic setting and associated mineralization. Exploratory airborne and ground geophysical surveys have been performed over Bear Lodge prospect, including gravity, induced-potential, controlled-source audio-magnetotellurics, and high-resolution aeromagnetic and radiometric surveys. The airborne datasets especially reveal previously unmapped dikes and numerous pipe-like bodies on the flanks of the dome. Results of this research are presented here as thematic maps and model visualizations.