THE ELK CREEK CARBONATITE COMPLEX

The Elk Creek carbonatite complex (ECCC) is a carbonatite-alkaline syenite complex located in southeast Nebraska. The carbonatite and related rocks are buried by more than 200 m of Pennsylvanian marine sedimentary rocks and Quaternary glacial till. The complex has been drilled extensively by Molycorp and more recently by Quantum Rare Earth Resources (now NioCorp Developments Ltd). The Molycorp drill core is now in the public domain. The rocks of the complex were intruded in Precambrian granite and gneiss on the eastern margin of the Mid-Continent rift where the rift has been offset by one of a series of southeasterly trending structures. The Pennsylvanian sub-crop is circular in plan view and 7 km in diameter. The primary rock type in the ECCC was described by Molycorp geologists as beforsite (dolomitic carbonatite). Other volumetrically important rock types with the ECCC include apatite beforsite and barite beforsite. Much of the apatite within the apatite beforsite is localized along fractures and in mm- to cm-scale zones of brecciation. Among the later rock types is a dolomite-magnetite-carbonatite (magnetite beforsite) phase that consists of two lobes, each approximately 0.1 square kilometer in size and extending downward to the maximum extent of drilling (900 meters). This magnetite beforsite is typically an aphanitic microbreccia including accessory apatite, barite, and pyrite. The magnetite is locally altered to hematite and other iron oxides ranging from a dusting of hematite to pervasive alteration to hematite and ferric oxyhydroxides. The magnetite oxidation occurs to depths of at least 630 m below the modern land surface. The thickness of the weathered regolith is typically only a few meters, however. Sulfide (predominantly pyrite with traces of chalcopyrite, sphalerite, and galena) and fluorite mineralization occurs in fractures in all carbonatite rock types but apparently pre-dates magnetite oxidation. The ECCC not only displays rare earth element (REE) enrichment but also is reported to be the largest Nb resource in the United States.

Our current research at Elk Creek is supported by the USGS Mineral Resource External Research Program.