NIOBIUM PYROCHLORE MINERALIZATION IN AN IRON-RICH DOLOMITE CARBONATITE, ELK CREEK, NEBRASKA (USA)

The Elk Creek Carbonatite (ECC) is an intrusive complex comprising carbonate and alkaline igneous rocks located in southeastern Nebraska. The complex is covered by Paleozoic sedimentary rocks and Quaternary glacial till. A Pennsylvanian subcrop map of the ECC is crudely circular in plan view, covering a maximum area of 49km² at the upper contact of the deposit with overlying sediments. The principle carbonate mineral in the ECC is dolomite with several mineral assemblages defined by dominant minor minerals such as apatite, barite, and magnetite (as determined by bulk geochemistry and petrography). The complex also contains silica-undersaturated carbonate-rich syenite and mafic lamprophyre dikes. Significant enrichment in niobium and rare-earth elements is present in the ECC, such that the deposit is a potential Nb-REE prospect.

The mineral assemblage of interest is an aphanitic dolomite microbreccia dominated by small euhedral-to-subhedral magnetite cubes forming disseminations, aggregates, and veins. Along with minor apatite and biotite, this assemblage generally resembles a phoscorite. Secondary hematite dusting on all mineral surfaces is common to pervasive. This assemblage exists as two “lobes” near the center of the ECC with a maximum plan view area of 0.1km² at approximately 700m below surface level. In individual core samples, intervals of magnetite dolomite are tens to hundreds of meters thick. These lobes are surrounded by a brecciated mix of magnetite dolomite and apatite-dominated dolomite. The magnetite dolomite is strongly associated with high niobium concentrations due to dense aggregates (occasionally disseminations) of very small (2-10 μm diameter) glassy cubic pyrochlore. Color is variable, mostly clear but occasionally pale green to pale tan. Zoning is not observed in petrographic thin-section. Aggregates occur in dolomite, but cathodoluminescent extinction suggests iron quenching where pyrochlore is present. Intergrowth with magnetite is occasionally seen at the margin of magnetite aggregates. Overall Nb concentration in bulk analysis is >10,000ppm with atypical depletion of light rare earths (La-Nd) compared to the other intrusive mineral assemblages in the Elk Creek deposit.