Elk Creek

A gravity and magnetic anomaly in southeastern Nebraska was drilled in 1971 and indicated 15 m of Quaternary sediments, underlain by Pennsylvanian limestones, shales and sandstones to a depth of about 200 m, which were followed by iron-rich carbonatites to the bottom of the hole at 286 m. The carbonatites do not appear to be rich enough in iron to account for the anomalies suggesting the presence of adjacent more iron-rich rocks (Brookins et al., 1975, p. 80). The carbonatites are variably brecciated or massive rocks of dolomite and ankerite, hematite, phlogopite, baryte, serpentine, probably after olivine, quartz and accessory pyrite, chalcopyrite, galena, sphalerite, feldspar, apatite and fluorite. Analyses on a range of samples indicate REE of 0.35-1.86%; P2O5, 2.2-5.0%; Nb2O5, 0.12-0.13%; Sr 40-2940 p.p.m.; Ba 228-598 p.p.m. and 87Sr/86Sr ratios of 0.7030-0.7085 (Brookins et al., 1975, Tables 1 and 2), apparently confirming these rocks as being carbonatite.