GEOPHYSICAL EXPRESSION OF THE ELK CREEK CARBONATITE, NEBRASKA: A BURIED NIOBIUM AND RARE EARTH ELEMENT DEPOSIT

The lower Paleozoic Elk Creek carbonatite is a 6-8 kilometer diameter intrusive complex buried under 200 meters of sedimentary rocks in southeastern Nebraska. It hosts the largest known niobium deposit in the U.S., and a rare earth element (REE) deposit. The carbonatite is made up of several lithologies that have poorly understood relationships with each other. Niobium mineralization is most enriched within a magnetite beforsite unit, and REE oxides are most concentrated in a barite beforsite unit. The carbonatite intrudes gneissic Proterozoic country rocks. Efforts to explore the carbonatite have relied upon geophysical data combined with drilling, and legacy cores and related data are publically available. A high-resolution airborne gravity gradient and magnetic survey was recently flown over the carbonatite. The carbonatite produces a vertical gravity gradient high with a subdued central low, and a central magnetic high surrounded by magnetic field values lower than those over the country rocks. The geophysical data, legacy borehole data, and new physical property data are combined for interpretation. The carbonatite is denser than the country rocks, explaining the vertical gravity gradient high. Most carbonatite lithologies have weaker magnetic susceptibilities than the country rocks, explaining why the carbonatite does not produce a magnetic high at its margin. The primary source of the central magnetic high is interpreted to be mafic rocks that are strongly magnetized and are known to be present in large volumes. Magnetite beforsite is very dense and strongly magnetized, producing a vertical gravity gradient high and contributing to the central aeromagnetic high. Barite beforsite, associated with the highest REE content, has physical properties that are similar to most of the carbonatite volume, making it a poor geophysical target. The hypothesized source of the central vertical gravity gradient low is a zone of intense alteration. The geophysical anomalies require the presence of dense and strongly magnetized rocks at depths greater than the deepest boreholes, either a large volume of magnetite beforsite or another unknown lithology. Magnetite beforsite occurs in an area where aeromagnetic lineaments intersect, indicating possible fault control on the unit’s location and niobium mineralization.