HIGH-RESOLUTION GEOPHYSICAL IMAGING OF A NORTH AMERICAN NIOBIUM AND RARE EARTH ELEMENT DEPOSIT: PRELIMINARY RESULTS FROM THE ELK CREEK CARBONATITE, NEBRASKA

The Paleozoic Elk Creek carbonatite is a 6-8 kilometer diameter complex buried under 200 meters of sedimentary rocks in southeastern Nebraska. It is the host of a niobium and rare earth element (REE) deposit. The carbonatite is made up of several different recognized lithologies that have poorly understood relationships with each other. Niobium mineralization is associated with a magnetite beforsite unit, and REE oxides are associated with a barite beforsite unit. The carbonatite intrudes gneissic country rocks. Efforts to explore the Elk Creek carbonatite have relied upon geophysical data combined with drilling, and 24 kilometers of core are stored by the University of Nebraska-Lincoln. A high-resolution airborne magnetic and gravity gradient survey was recently flown over the carbonatite. The carbonatite produces a 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 and physical property information extracted from the core make a strong combination for interpretation. Preliminary conclusions are derived from joint analysis of those data: 1) The carbonatite is denser than the country rocks, explaining the gravity gradient high. 2) 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 significant volumes. 3) The magnetite beforsite lithology, associated with niobium, is dense and strongly magnetized. It is a robust geophysical target if present in a large enough volume, but its known volume is small and its contribution to geophysical anomalies is uncertain. 4) The barite beforsite unit, associated with REEs, has physical properties that are similar to most of the carbonatite volume, making it a poor geophysical target. 5) The hypothesized source of the central gravity gradient low is a zone of alteration. 6) A gravity field high over the south-central part of the carbonatite may be explained by a significant volume of deeply buried magnetite beforsite, or another lithology not known from boreholes.